Docker Build Strategies

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 git@github.com: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).