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Docker Build Strategies

DockerOfficialPublished by Docker

  • Skill
  • Tool integration
  • Cloud and DevOps
  • Apache-2.0
  • Medium risk
  • Has scripts
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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://mp-staging.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

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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

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