AutoGineer
Generated banner for Docker Build Strategies
Generated logo for Docker Build Strategies

Docker Build Strategies

DockerOfficialPublished by Docker

  • Skill
  • Tool integration
  • Cloud and DevOps
  • Apache-2.0
  • Medium risk
  • Has scripts
Download ZIP

From the author

Use this skill when writing, reviewing, or optimizing Dockerfiles, even if the user just says their image is too large, their build is slow, or they need to harden a container for production. Covers multi-stage builds, layer caching, .dockerignore, non-root users, and image size optimization.
— the description by Docker

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

This skill offers guidance on creating and maintaining Dockerfiles. It focuses on implementing multi-stage builds, managing layer caching, securing containers with non-root users, and optimizing image sizes. It provides specific patterns for Go, Node.js, and Python, and includes scripts for verifying build configurations.

What it does

  • Provides patterns for multi-stage builds to separate build-time dependencies from runtime images.
  • Explains how to use BuildKit cache mounts to speed up dependency installation.
  • Offers guidance on using BuildKit secrets and SSH mounts to handle credentials securely.
  • Defines rules for configuring non-root users and setting file ownership.
  • Includes a script to verify image size and user configuration.
  • Provides a comprehensive template for .dockerignore files.

When to use it

  • When writing a new Dockerfile for a project.
  • When optimizing an existing Dockerfile for image size or build speed.
  • When hardening a container for production use.
  • When reviewing a Dockerfile for adherence to build best practices.

Advantages

  • Reduces image size by using multi-stage builds and minimal base images.
  • Improves build speed through effective layer caching and cache mounts.
  • Enhances security by avoiding credential persistence in image layers.
  • Ensures containers run with restricted privileges using non-root users.

Usage examples

“How can I make my Docker image smaller?”
The agent will suggest using multi-stage builds, selecting minimal base images like distroless or Alpine, and using .dockerignore to exclude unnecessary files.
“My Docker build is slow, how do I fix it?”
The agent will recommend reordering instructions to maximize layer caching and implementing BuildKit cache mounts for package managers.
“How do I securely pass a private npm token to my Docker build?”
The agent will provide instructions on using --mount=type=secret to access the token during the build without baking it into the image layers.
“How do I run my container as a non-root user?”
The agent will provide the Dockerfile instructions to create a system user and group, and set the appropriate ownership for application files.

Install

  1. Download the skill as a ZIP:

    https://autogineer.com/agent-skills/docker/docker-build-strategies/download

  2. Unzip it into the cross-client skills folder of your project:

    mkdir -p .agents/skills && unzip docker-build-strategies.zip -d .agents/skills

Documentation by Docker

Docker Build Strategies

Overview

This skill provides rules and patterns for writing and reviewing production-quality Dockerfiles. Apply it when the main task is image-build quality: multi-stage builds, cache behavior, non-root execution, build context hygiene, and runtime image size.

When to use this skill

Activate this skill when:

  • Creating a new Dockerfile for any language or framework

  • Optimizing an existing Dockerfile for size, speed, or security

  • Reviewing a Dockerfile for best-practice compliance

  • Adding a .dockerignore file to a project

Do not use this skill when

Do not use this skill when:

  • The project has no Docker setup yet and the main need is a first-pass scaffold

  • The main task is wiring services together in compose.yaml

  • The main task is debugging Compose startup ordering, networking, or development overrides

Core guidance

Multi-stage builds

Use multi-stage builds when the project has a build step or when build-time dependencies differ from runtime. Separate build-time dependencies from the runtime image.

  1. Name every stage explicitly (FROM ... AS build, FROM ... AS runtime).

  2. Use the smallest appropriate base for the runtime stage: distroless, alpine, or slim variants.

  3. Copy only the final artifact into the runtime stage with COPY --from=build.

  4. Use COPY --link when copying from a prior stage or adding static files — it improves cache reuse by making the COPY independent of previous layers.

See references/multi-stage-builds.md for language-specific patterns (Go, Node, Python, Java).

Layer caching

Order Dockerfile instructions from least-frequently-changed to most-frequently-changed.

  1. Place dependency manifests (package.json, go.mod, requirements.txt) and install steps before copying application source code. Bind-mount the manifest into the install step instead of COPY-ing it, so it never enters a layer: RUN --mount=type=bind,source=package.json,target=package.json --mount=type=bind,source=package-lock.json,target=package-lock.json npm ci. This is safe for install commands that only read the manifest (npm ci, pip install -r, go mod download); if a step also needs to write the manifest back into the image, COPY it instead.

  2. Use BuildKit cache mounts for package manager caches:

    • Go: RUN --mount=type=cache,target=/go/pkg/mod go build ...

    • Node: RUN --mount=type=cache,target=/root/.npm npm ci

    • Python: RUN --mount=type=cache,target=/root/.cache/pip pip install ...

    • apt: RUN --mount=type=cache,target=/var/cache/apt,sharing=locked --mount=type=cache,target=/var/lib/apt,sharing=locked apt-get update && apt-get install -y ... — no rm -rf /var/lib/apt/lists/* needed, since the cache lives outside the image layer. sharing=locked is required because apt needs exclusive access to its cache directories.

    • apk (Alpine — per the Alpine wiki, not a Docker-verified doc; references/layer-caching.md links the source): RUN --mount=type=cache,target=/etc/apk/cache,sharing=locked apk add ... — drop --no-cache so downloaded packages land in the mounted cache directory instead of being discarded.

  3. Pin base image tags to a specific version or digest — never use latest in production.

  4. Combine related RUN commands with && to reduce layer count, but keep logically distinct steps separate for cache granularity.

See references/layer-caching.md for detailed cache invalidation rules and cache mount patterns.

Build secrets and SSH access

Never bake credentials into the image. Use BuildKit secrets and SSH mounts so credentials are available only during the specific RUN step that needs them, and never persist in any layer or docker history output.

  1. Do NOT pass credentials through ARG or ENV. Both end up in the image layers and are inspectable via docker history.

  2. Do NOT COPY credential files into the build context: .npmrc, .pypirc, .netrc, pip.conf, Maven settings.xml, .env, cloud credentials (~/.aws/credentials, ~/.config/gcloud/, service-account JSON files, ~/.azure/), secret-manager tokens (~/.vault-token), package-registry tokens (~/.cargo/credentials.toml), TLS keys (*.pem, *.p12), kubeconfig, SSH keys (id_rsa, id_dsa, id_ed25519, id_ecdsa). Even when the final stage does not copy them forward, they live in intermediate layers and the build cache.

  3. Do NOT echo, write, or expand the secret value inside a RUN command in a way that persists it to a layer or emits it to build logs. Access the secret file (e.g., /run/secrets/<id>, or directly via the mount target=) — never echo "$(cat /run/secrets/X)", never substitute it into a shell argument that will be logged with --progress=plain.

  4. Use RUN --mount=type=secret for package manager registry credentials:

    RUN --mount=type=secret,id=npmrc,target=/root/.npmrc,required=false \
    --mount=type=cache,target=/root/.npm \
    npm ci --omit=dev

    The secret is available only inside that RUN, never written to a layer. Use required=true when the build will always need the credential (e.g., all packages come from a private registry, so missing the secret should fail the build immediately); use required=false only when the secret is optional (the build can succeed with public packages alone).

  5. Use RUN --mount=type=ssh for fetching private Git repositories or modules. The build container has no known_hosts by default — populate it inside the same RUN:

    RUN --mount=type=ssh \
    mkdir -p -m 0700 /root/.ssh && \
    ssh-keyscan github.com >> /root/.ssh/known_hosts && \
    git clone [email protected]:org/private-repo.git

    Do NOT use StrictHostKeyChecking=no as a shortcut — it disables host-key verification entirely. ssh-keyscan accepts whatever host key the server presents each time the step runs; nothing is pinned between builds. For stronger assurance, compare it against the provider's published host key fingerprints, or write the published key into known_hosts instead of scanning.

  6. Invoke buildx with the secret and SSH sources:

    # --ssh default forwards this shell's SSH agent (SSH_AUTH_SOCK); list every key the build can use:
ssh-add -l

docker buildx build \
    --secret id=npmrc,src=$HOME/.npmrc \
    --ssh default \
    .

    The RUN --mount=type=ssh step can use every key that ssh-add -l lists, so expose only the key this build needs. In an interactive terminal, run ssh-agent bash to start a shell with a dedicated agent, then run ssh-add <key-file>, confirm that ssh-add -l lists only that key, and run the build in that shell. A tool that starts a new shell for each command loses that agent between commands, so ask the user to run these steps. Alternatively, pass an unencrypted key file, such as a dedicated deploy key, directly with --ssh default=<key-file>; BuildKit rejects passphrase-protected keys in this form, so load those into an agent instead.

  7. .dockerignore exclusions of .env and credential files are defense in depth, not the primary mechanism — keep them, but do not rely on them as your only protection.

See references/multi-stage-builds.md for per-language patterns (npm, pip, Maven, Go GOPRIVATE).

.dockerignore

Always generate a .dockerignore alongside the Dockerfile. Exclude:

  • .git/, .github/, .vscode/, .idea/

  • node_modules/, __pycache__/, .venv/, vendor/ (when rebuilt in the build stage)

  • *.md, LICENSE, docs/

  • Build outputs, test artifacts, and IDE configs

  • .env files and any secrets

See assets/dockerignore-example for a comprehensive template.

Non-root user

Always configure the final image to run as a non-root user.

  1. Create a dedicated user and group in the runtime stage:

    RUN addgroup --system --gid 1001 appgroup && \
    adduser --system --uid 1001 --ingroup appgroup appuser

  2. Set ownership on application files: COPY --from=build --chown=appuser:appgroup /app /app

  3. When combining --chown with COPY --link, always use the numeric UID:GID you assigned (e.g., --chown=1001:1001 if you used --uid 1001 --gid 1001 above), not named users. --link creates an independent layer where named users from prior RUN instructions are not available.

  4. Place the USER appuser instruction after all file operations and before ENTRYPOINT/CMD.

  5. On distroless images, use the built-in nonroot user: USER nonroot:nonroot.

Image size optimization

  1. Prefer FROM scratch (Go static binaries), distroless, or Alpine-based images for the runtime stage.

  2. Install OS packages with a BuildKit cache mount rather than rm -rf-ing the cache in the same layer — see "Layer caching" above. The cache mount keeps the package cache out of the image layer entirely, so no cleanup step is needed.

  3. Do not install documentation, man pages, or debug tools in the runtime image.

  4. Use .dockerignore aggressively to minimize the build context.

General rules

  • Always include a # syntax=docker/dockerfile:1 directive as the first line to enable BuildKit features.

  • Set WORKDIR before any COPY or RUN instructions — never rely on the default /.

  • Prefer ENTRYPOINT with exec form (["binary"]) over shell form.

  • Add EXPOSE to document the listening port.

  • Add metadata labels: LABEL org.opencontainers.image.source=...

Related skills

  • For first-time Docker project scaffolding and deciding which files to create, use docker-project-foundations.

  • For service dependencies, health checks, overrides, networks, and volume patterns, use docker-compose-patterns.

  • For destructive Docker CLI commands (docker system prune, docker rm -f, image/network/builder pruning) and a cross-product index of destructive-command guardrails, use docker-destructive-guardrails.

References

  • references/multi-stage-builds.md — Language-specific multi-stage patterns for Go, Node.js, Python, and Java

  • references/layer-caching.md — Deep dive on layer ordering, cache invalidation, and BuildKit cache mounts

Assets

  • assets/Dockerfile.go — Multi-stage Go build with distroless runtime and non-root user

  • assets/Dockerfile.nodejs — Multi-stage Node.js build with proper layer caching and non-root user

  • assets/Dockerfile.python — Python build with virtual env, layer ordering, and non-root user

  • assets/dockerignore-example — Comprehensive .dockerignore template

Scripts

  • scripts/verify-build.sh — Builds the Dockerfile in the current directory, then reports image size and configured user. Run it from the project root (the directory that contains the Dockerfile), with the script path resolved under this skill's directory:

    bash "<skill-dir>/scripts/verify-build.sh" [--help] [IMAGE_NAME]

    Replace <skill-dir> with the absolute path of the folder that contains this SKILL.md; the scripts/ path is relative to that folder, not to the project. Do not change into the skill directory first: the script builds whatever is in the current directory. If the skill directory cannot be resolved, run docker build -t verify-build-test ., then docker images verify-build-test and docker inspect verify-build-test --format '{{.Config.User}}'. Exit status is 0 when all Docker commands succeed or help is requested, the failing Docker command's non-zero status when verification fails, and 2 for invalid arguments.

Checks

  • checks/verification.md — Detailed verification runbook for manual review.

Files

LICENSE


                                 Apache License
                           Version 2.0, January 2004
                        http://www.apache.org/licenses/

   TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION

   1. Definitions.

      "License" shall mean the terms and conditions for use, reproduction,
      and distribution as defined by Sections 1 through 9 of this document.

      "Licensor" shall mean the copyright owner or entity authorized by
      the copyright owner that is granting the License.

      "Legal Entity" shall mean the union of the acting entity and all
      other entities that control, are controlled by, or are under common
      control with that entity. For the purposes of this definition,
      "control" means (i) the power, direct or indirect, to cause the
      direction or management of such entity, whether by contract or
      otherwise, or (ii) ownership of fifty percent (50%) or more of the
      outstanding shares, or (iii) beneficial ownership of such entity.

      "You" (or "Your") shall mean an individual or Legal Entity
      exercising permissions granted by this License.

      "Source" form shall mean the preferred form for making modifications,
      including but not limited to software source code, documentation
      source, and configuration files.

      "Object" form shall mean any form resulting from mechanical
      transformation or translation of a Source form, including but
      not limited to compiled object code, generated documentation,
      and conversions to other media types.

      "Work" shall mean the work of authorship, whether in Source or
      Object form, made available under the License, as indicated by a
      copyright notice that is included in or attached to the work
      (an example is provided in the Appendix below).

      "Derivative Works" shall mean any work, whether in Source or Object
      form, that is based on (or derived from) the Work and for which the
      editorial revisions, annotations, elaborations, or other modifications
      represent, as a whole, an original work of authorship. For the purposes
      of this License, Derivative Works shall not include works that remain
      separable from, or merely link (or bind by name) to the interfaces of,
      the Work and Derivative Works thereof.

      "Contribution" shall mean any work of authorship, including
      the original version of the Work and any modifications or additions
      to that Work or Derivative Works thereof, that is intentionally
      submitted to Licensor for inclusion in the Work by the copyright owner
      or by an individual or Legal Entity authorized to submit on behalf of
      the copyright owner. For the purposes of this definition, "submitted"
      means any form of electronic, verbal, or written communication sent
      to the Licensor or its representatives, including but not limited to
      communication on electronic mailing lists, source code control systems,
      and issue tracking systems that are managed by, or on behalf of, the
      Licensor for the purpose of discussing and improving the Work, but
      excluding communication that is conspicuously marked or otherwise
      designated in writing by the copyright owner as "Not a Contribution."

      "Contributor" shall mean Licensor and any individual or Legal Entity
      on behalf of whom a Contribution has been received by Licensor and
      subsequently incorporated within the Work.

   2. Grant of Copyright License. Subject to the terms and conditions of
      this License, each Contributor hereby grants to You a perpetual,
      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
      copyright license to reproduce, prepare Derivative Works of,
      publicly display, publicly perform, sublicense, and distribute the
      Work and such Derivative Works in Source or Object form.

   3. Grant of Patent License. Subject to the terms and conditions of
      this License, each Contributor hereby grants to You a perpetual,
      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
      (except as stated in this section) patent license to make, have made,
      use, offer to sell, sell, import, and otherwise transfer the Work,
      where such license applies only to those patent claims licensable
      by such Contributor that are necessarily infringed by their
      Contribution(s) alone or by combination of their Contribution(s)
      with the Work to which such Contribution(s) was submitted. If You
      institute patent litigation against any entity (including a
      cross-claim or counterclaim in a lawsuit) alleging that the Work
      or a Contribution incorporated within the Work constitutes direct
      or contributory patent infringement, then any patent licenses
      granted to You under this License for that Work shall terminate
      as of the date such litigation is filed.

   4. Redistribution. You may reproduce and distribute copies of the
      Work or Derivative Works thereof in any medium, with or without
      modifications, and in Source or Object form, provided that You
      meet the following conditions:

      (a) You must give any other recipients of the Work or
          Derivative Works a copy of this License; and

      (b) You must cause any modified files to carry prominent notices
          stating that You changed the files; and

      (c) You must retain, in the Source form of any Derivative Works
          that You distribute, all copyright, patent, trademark, and
          attribution notices from the Source form of the Work,
          excluding those notices that do not pertain to any part of
          the Derivative Works; and

      (d) If the Work includes a "NOTICE" text file as part of its
          distribution, then any Derivative Works that You distribute must
          include a readable copy of the attribution notices contained
          within such NOTICE file, excluding those notices that do not
          pertain to any part of the Derivative Works, in at least one
          of the following places: within a NOTICE text file distributed
          as part of the Derivative Works; within the Source form or
          documentation, if provided along with the Derivative Works; or,
          within a display generated by the Derivative Works, if and
          wherever such third-party notices normally appear. The contents
          of the NOTICE file are for informational purposes only and
          do not modify the License. You may add Your own attribution
          notices within Derivative Works that You distribute, alongside
          or as an addendum to the NOTICE text from the Work, provided
          that such additional attribution notices cannot be construed
          as modifying the License.

      You may add Your own copyright statement to Your modifications and
      may provide additional or different license terms and conditions
      for use, reproduction, or distribution of Your modifications, or
      for any such Derivative Works as a whole, provided Your use,
      reproduction, and distribution of the Work otherwise complies with
      the conditions stated in this License.

   5. Submission of Contributions. Unless You explicitly state otherwise,
      any Contribution intentionally submitted for inclusion in the Work
      by You to the Licensor shall be under the terms and conditions of
      this License, without any additional terms or conditions.
      Notwithstanding the above, nothing herein shall supersede or modify
      the terms of any separate license agreement you may have executed
      with Licensor regarding such Contributions.

   6. Trademarks. This License does not grant permission to use the trade
      names, trademarks, service marks, or product names of the Licensor,
      except as required for reasonable and customary use in describing the
      origin of the Work and reproducing the content of the NOTICE file.

   7. Disclaimer of Warranty. Unless required by applicable law or
      agreed to in writing, Licensor provides the Work (and each
      Contributor provides its Contributions) on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
      implied, including, without limitation, any warranties or conditions
      of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A
      PARTICULAR PURPOSE. You are solely responsible for determining the
      appropriateness of using or redistributing the Work and assume any
      risks associated with Your exercise of permissions under this License.

   8. Limitation of Liability. In no event and under no legal theory,
      whether in tort (including negligence), contract, or otherwise,
      unless required by applicable law (such as deliberate and grossly
      negligent acts) or agreed to in writing, shall any Contributor be
      liable to You for damages, including any direct, indirect, special,
      incidental, or consequential damages of any character arising as a
      result of this License or out of the use or inability to use the
      Work (including but not limited to damages for loss of goodwill,
      work stoppage, computer failure or malfunction, or any and all
      other commercial damages or losses), even if such Contributor
      has been advised of the possibility of such damages.

   9. Accepting Warranty or Additional Liability. While redistributing
      the Work or Derivative Works thereof, You may choose to offer,
      and charge a fee for, acceptance of support, warranty, indemnity,
      or other liability obligations and/or rights consistent with this
      License. However, in accepting such obligations, You may act only
      on Your own behalf and on Your sole responsibility, not on behalf
      of any other Contributor, and only if You agree to indemnify,
      defend, and hold each Contributor harmless for any liability
      incurred by, or claims asserted against, such Contributor by reason
      of your accepting any such warranty or additional liability.

   END OF TERMS AND CONDITIONS

   APPENDIX: How to apply the Apache License to your work.

      To apply the Apache License to your work, attach the following
      boilerplate notice, with the fields enclosed by brackets "[]"
      replaced with your own identifying information. (Don't include
      the brackets!)  The text should be enclosed in the appropriate
      comment syntax for the file format. We also recommend that a
      file or class name and description of purpose be included on the
      same "printed page" as the copyright notice for easier
      identification within third-party archives.

   Copyright 2026 Docker, Inc.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.

Requirements and compatibility

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Needs: Needs a command-line tool, Needs Python

The author says: Requires Docker 23.0+ (BuildKit default). On Docker 20.10–22.x, set DOCKER_BUILDKIT=1 before building.

Security

What our scanner noticed· Medium risk

  • References to credential files
  • Instructions that use the network
  • Scripts that your agent may run

This skill includes scripts that your agent may run. Read them before you use it.

A static scan is not a guarantee. Skills are third-party content: review a skill before you let an agent use it.

Attribution and licence

Published by
Docker
Publisher
Docker (Official)
Source
docker/skills · skills/docker-build-strategies · commit 9f04a05
Licence
Apache License 2.0 (redistribution allowed)
  • LICENSE (added by AutoGineer from LICENSE)
  • ATTRIBUTION.md (written by AutoGineer)
Synced

AutoGineer is not affiliated with or endorsed by Docker or Docker.

Download ZIP

The ZIP holds the skill’s files, its licence files and an ATTRIBUTION.md. Download docker-build-strategies.zip