docker-guide

name	docker-guide
description	Use when writing or editing Docker images and Compose files for production. Triggers on `Dockerfile`, `docker-compose.yml`, `.dockerignore`, and prompts about multi-stage builds, alpine/distroless base images, layer caching, non-root users, BuildKit, secrets, health checks, or image slimming, even when the user doesn't say 'Docker'.

Docker Coding Guidelines

Essentials

• Build optimization - Multi-stage builds, small base images (alpine/distroless), see references/multi-stage-builds.md
• Layer caching - Order layers for caching, copy lockfiles before source, see references/layer-caching.md
• Security - Run as non-root, least privilege, no secrets in images, see references/security.md
• Configuration - Externalize config via env/volumes, pin versions, see references/production-config.md
• Quality - Use BuildKit and hadolint, add health endpoints, see references/production-config.md

Gotchas

• Layer cache invalidates on the first changed line of a COPY/RUN — order layers from least-to-most-frequently-changed (deps before source)
• RUN apt-get install … && rm -rf /var/lib/apt/lists/* must be one layer — separate RUNs leave the apt cache in an earlier layer permanently
• .dockerignore is critical for build context size and to avoid leaking secrets — node_modules/ and .git/ aren't excluded by default
• Multi-stage builds need explicit COPY --from=<stage> — forgetting it copies the whole heavy stage into the final image
• ENTRYPOINT + CMD interact: CMD provides default args to ENTRYPOINT; overriding CMD from docker run drops them, not appends

Progressive disclosure

• Read references/multi-stage-builds.md - Load when creating production images or optimizing build process
• Read references/layer-caching.md - Load when builds are slow or cache invalidates frequently
• Read references/security.md - Load when hardening images or handling secrets
• Read references/production-config.md - Load when adding health checks or configuring for orchestration
• Read references/docker-compose.md - Load when defining multi-service local development environments