Docker Build Strategies Agent Skill

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.

• Name every stage explicitly (FROM ... AS build, FROM ... AS runtime).
• Use the smallest appropriate base for the runtime stage: distroless, alpine, or slim variants.
• Copy only the final artifact into the runtime stage with COPY --from=build.
• 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.

• Place dependency manifests (package.json, go.mod, requirements.txt) and install steps before copying application source code.
• 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 ...
• Pin base image tags to a specific version or digest — never use latest in production.
• 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.

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

• 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 pins the known fingerprint at build time.

• Invoke buildx with the secret and SSH sources:

   # Ensure an SSH agent is running with the key loaded (or use --ssh default=<key-file>):
   eval "$(ssh-agent -s)" && ssh-add ~/.ssh/id_ed25519

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

Alternatively, pass the key file directly without an agent: --ssh default=$HOME/.ssh/id_ed25519.

• .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.

• Create a dedicated user and group in the runtime stage:

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

• Set ownership on application files: COPY --from=build --chown=appuser:appgroup /app /app
• 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.
• Place the USER appuser instruction after all file operations and before ENTRYPOINT/CMD.
• On distroless images, use the built-in nonroot user: USER nonroot:nonroot.

Image size optimization

• Prefer FROM scratch (Go static binaries), distroless, or Alpine-based images for the runtime stage.
• Remove package manager caches in the same RUN layer that installs packages: apt-get install -y ... && rm -rf /var/lib/apt/lists/*
• Do not install documentation, man pages, or debug tools in the runtime image.
• 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 image, reports size and configured user.

  bash scripts/verify-build.sh [--help] [IMAGE_NAME]

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.