Dockerfile Pro

dockerfile-pro

Overview

Produce production-grade Dockerfiles that are small (minimal final image), secure (non-root, pinned, no secrets, minimal surface), and reproducible (deterministic builds with effective layer caching). Applies to any language runtime (Go, Rust, Node, Python, Java, .NET, etc.) and any registry.

Keywords: Dockerfile, container, multi-stage build, build cache, BuildKit, non-root user, distroless, alpine, slim, image size, layer caching, .dockerignore, HEALTHCHECK, OCI image, supply chain, reproducible build, hadolint, Trivy.

The three goals (small / secure / reproducible) often align, but when they conflict, prioritize security > reproducibility > size.

Workflow

Follow these steps in order when authoring or reviewing a Dockerfile.

1. Identify the app shape. Compiled binary (Go/Rust/C++) → use a FROM scratch or distroless final stage. Interpreted/runtime (Node/Python/Java/.NET) → use the matching -slim/distroless runtime image. JVM/CLR may need a JRE-only final image.
2. Always use multi-stage. A builder stage installs toolchains and compiles/installs dependencies; the final stage copies only the runtime artifacts. Never ship compilers, package caches, or dev headers.
3. Pin everything. Pin the base image by tag and digest (image:tag@sha256:...). Pin OS and language package versions. This is what makes builds reproducible.
4. Order layers from least- to most-frequently-changing. Copy dependency manifests (package.json, go.mod, requirements.txt, *.csproj) and install deps before copying application source. Source changes then don't bust the dependency layer.
5. Use BuildKit cache mounts (RUN --mount=type=cache,...) for package-manager caches so repeated builds are fast without bloating the image.
6. Drop to a non-root user in the final stage with an explicit numeric UID/GID. Make the filesystem read-only-friendly.
7. Minimize layers and surface. Combine related RUN steps, clean package caches in the same layer, and copy with --chown instead of a separate chown layer.
8. Add operational metadata. WORKDIR, EXPOSE, ENV, OCI LABELs, HEALTHCHECK, and an exec-form ENTRYPOINT/CMD.
9. Add a .dockerignore. Exclude .git, node_modules, build output, secrets, and CI files so the build context stays small and secrets never enter an image.
10. Lint and scan. Run hadolint Dockerfile and a vulnerability scan (e.g. trivy image). See scripts/check_dockerfile.py for a fast static audit you can run with zero dependencies.

Decision frameworks

Which final base image?

Rule of thumb: distroless or scratch for compiled apps; -slim for interpreted apps. Avoid full :latest / fat base images.

Alpine vs slim?

• Alpine: smallest, but musl libc can break native deps (Python wheels, glibc binaries) and complicate DNS/locale handling.
• Debian -slim: slightly larger but maximally compatible. Default to -slim unless you have measured a real size win and tested compatibility.

Best Practices

• Pin base by digest: FROM node:22.11.0-slim@sha256:<digest>. Tags are mutable; digests are not.
• One concern per stage, named with AS builder, AS deps, AS runtime.
• Exec form only: CMD ["node","server.js"] — never the shell form CMD node server.js (it breaks signal handling and PID 1 semantics).
• Non-root with numeric UID: USER 10001:10001. A numeric UID lets Kubernetes enforce runAsNonRoot even without /etc/passwd.
• COPY --chown=10001:10001 to set ownership without an extra layer; never chmod -R 777.
• Clean in the same layer: apt-get update && apt-get install -y --no-install-recommends X && rm -rf /var/lib/apt/lists/*.
• --no-install-recommends (apt) / --no-cache (apk) / --frozen-lockfile (npm ci) for determinism.
• Pass secrets via RUN --mount=type=secret, never ARG/ENV (they persist in image history and docker history leaks them).
• Add HEALTHCHECK for long-running services so orchestrators can detect liveness.
• Set WORKDIR explicitly; never rely on /.
• Use tini or --init (or a runtime that reaps zombies) when your process spawns children.
• See references/best-practices.md for the full annotated checklist and rationale.

Common Pitfalls

• ❌ Running as root in the final image (the default). Always add USER.
• ❌ ADD for local files — use COPY. Reserve ADD for remote URLs/tar auto-extraction (and prefer not to).
• ❌ Secrets in ARG/ENV or copied .env files — they leak via docker history and image layers.
• ❌ apt-get upgrade / unpinned latest — destroys reproducibility.
• ❌ Separate RUN chmod/chown after COPY — doubles the data on disk across layers. Use COPY --chown.
• ❌ Copying the whole context first (COPY . .) before installing deps — busts the cache on every source edit.
• ❌ Shell-form CMD/ENTRYPOINT — your process won't receive SIGTERM, so graceful shutdown breaks.
• ❌ No .dockerignore — bloats context, slows builds, risks leaking .git/credentials.
• ❌ Shipping the build toolchain in the final image — keep it in the builder stage only.

Supporting files

• references/best-practices.md — exhaustive, annotated best-practice checklist with rationale, BuildKit cache-mount and secret-mount recipes, and a security/supply-chain section.
• examples/go-multistage.Dockerfile — minimal scratch-based Go image (non-root, pinned, static).
• examples/node-multistage.Dockerfile — Node.js multi-stage with npm ci, cache mounts, distroless final.
• examples/python-multistage.Dockerfile — Python slim with venv copy and non-root user.
• templates/Dockerfile.template — language-agnostic fill-in-the-blanks multi-stage template.
• templates/dockerignore.template — sensible default .dockerignore.
• scripts/check_dockerfile.py — zero-dependency static auditor that flags root users, unpinned bases, secrets in ARG, shell-form CMD, missing .dockerignore, and more.

Use the examples/templates as the starting skeleton, then apply the workflow and run scripts/check_dockerfile.py plus hadolint before finishing.