Writing Lean Dockerfiles and Shell Entrypoints

In production environments, every megabyte in a Docker image has a cost: slower pulls, larger attack surfaces, and wasted registry storage. Lean Dockerfiles combined with robust shell entrypoints are a hallmark of mature DevOps practice.

• Multi-stage builds separate build-time tooling from the final runtime image, dramatically reducing size.
• Entrypoint shims are small shell scripts that bootstrap containers: they template config files, validate environment variables, handle signals, and finally exec the main process.
• Together they form the backbone of reliable, portable container workloads in Kubernetes, ECS, and bare-metal environments.

This lesson covers both disciplines end-to-end, with production-grade patterns you can drop directly into your pipelines.

A multi-stage Dockerfile uses multiple FROM instructions. Each stage is an isolated layer set; you copy only the artifacts you need into the next stage.

• Stage 0 (builder): installs compilers, test runners, build dependencies.
• Stage 1 (runtime): starts from a minimal base (e.g. alpine, distroless) and copies only compiled binaries or app bundles.
• The final image never contains gcc, make, or source code unless you explicitly copy them.

Use --from=<stage> in COPY to pull files across stage boundaries. Name stages with AS <name> for readability and selective targeting with docker build --target.