Docker

Project-specific containerization patterns for Dockerfile and Docker Compose.

Architecture Decisions

Image Building

1. Minimal base images — Use slim/alpine variants; pin to digest for reproducibility.
2. Multi-stage builds — Separate build dependencies from runtime.
3. Layer optimization — Combine RUN commands; place frequently changed files last.
4. COPY over ADD — ADD only for tar extraction or remote URLs.

Security

1. Non-root users — Always use UID >10000; never run as root in production.
2. No secrets in images — Use Docker secrets or runtime env injection.
3. .dockerignore required — Exclude .git, .env, node_modules, build artifacts.

Runtime

1. One process per container — Single responsibility principle.
2. Healthchecks required — Define HEALTHCHECK in Dockerfile or Compose.
3. Resource limits — Always set mem_limit and cpus in production.

Compose

1. Network segmentation — Dedicated networks per service group.
2. Named volumes — Never use anonymous volumes in production.
3. depends_on with healthchecks — Use condition: service_healthy.
4. Environment separation — Use override files for dev/staging/prod.

Gotchas

• COPY . . before RUN npm install busts the cache on EVERY code change. Copy package*.json first, install, THEN copy source.
• Alpine uses musl libc, not glibc. Python packages with C extensions (numpy, pandas, cryptography) may fail to install or need apk add build dependencies. Consider -slim variants if you hit this.
• ENTRYPOINT ["python", "app.py"] (exec form) handles signals correctly. ENTRYPOINT python app.py (shell form) wraps in /bin/sh -c and PID 1 won't receive SIGTERM — containers take 10s to stop.
• Docker layer cache is invalidated from the FIRST changed layer downward. A changed COPY near the top rebuilds everything below it.
• depends_on without condition: service_healthy only waits for container START, not readiness. Your app will crash connecting to a database that's still initializing.
• host.docker.internal works on Docker Desktop (Mac/Windows) but NOT on Linux. Use --network host or explicit container networking on Linux.
• Build args (ARG) are NOT available after FROM in multi-stage builds unless re-declared. Each stage starts fresh.
• docker compose up reuses existing containers. After changing Dockerfile, you need docker compose up --build or docker compose build first.
• Volume mounts override the container's filesystem — if your node_modules are built inside the container but you mount .:/app, the host's (possibly empty) node_modules shadows them. Use a named volume for node_modules.
• EXPOSE is documentation only — it does NOT publish the port. You still need -p 8080:8080 or ports: in compose.
• Docker's default bridge network does NOT provide DNS resolution between containers. Use a custom network or compose's default network.

References