docker-expertlisted

# Docker Expert Build small, reproducible, non-root Docker images using Docker 27+ and BuildKit. ## When to Use This Skill - Creating or changing a `Dockerfile` - Creating or debugging a `docker-compose.yml` or `compose.yaml` - Converting a single-stage image into a multi-stage production image - Improving Docker build cache behavior or BuildKit usage - Adding `.dockerignore`, health checks, or non-root execution - Diagnosing Docker image-size, startup, networking, volume, or Compose issues ## Core Workflow 1. **Inspect the runtime contract** - identify the app start command, listening port, required files, environment variables, native dependencies, and health endpoint. Read existing Dockerfiles, Compose files, lockfiles, and `.dockerignore` before making changes. 2. **Design build and runtime stages** - separate build dependencies from runtime dependencies. Pin an explicit maintained base-image version, use named build stages, and copy only required runtime artifacts into the final image. 3. **Implement Dockerfile and ignore rules** - order layers from least frequently changed to most frequently changed: base image, OS packages, dependency manifests, dependency installation, then application source. Add `.dockerignore` before relying on `COPY . .`. 4. **Verify the image build** - run `docker build --progress=plain -t app:local .`; fix every reported issue and re-run until clean. Change only an application source file, rebuild, and confirm dependency installation is