Building small, reproducible container images

Goal

Produce an image whose contents are fully determined by the repository, small enough to move quickly, and safe to run.

Prerequisites

A locked dependency set and a clear separation between build-time and run-time needs.

Steps

1. Start FROM an official, slim base image pinned by digest (image:tag@sha256:…), and record when it was last updated.
2. Order instructions from least to most frequently changing (dependencies before source) so that layer caching works.
3. Install dependencies from the lockfile with hash verification; do not run package managers' "latest" resolution inside the build.
4. Use a multi-stage build when compilation is needed: build tools stay in the builder stage; the final stage copies artifacts only.
5. Copy only the runtime files with a .dockerignore that excludes tests, caches, secrets and the .git directory.
6. Create and switch to a non-root USER, set HEALTHCHECK, and define the command in exec form.
7. Never pass secrets through ARG or bake them into layers; they remain in the image history.

Expected result

Two builds of the same commit produce equivalent images; the image contains no build tools, secrets or test data; the container starts unprivileged and reports its health.

Limits and test basis

Digest pinning must be refreshed deliberately to receive base-image security updates. Distroless or scratch images reduce attack surface but complicate debugging. Practices follow the cited documentation.