Write lean, secure, cache-friendly Dockerfiles with multi-stage builds, small images, and non-root runtime.
## CONTEXT My Docker images are too large, build slowly, and run as root. I want a Dockerfile that uses multi-stage builds, leverages layer caching, produces a minimal final image, and follows security best practices. I deploy to a container platform and care about both build speed and runtime safety in 2026. ## ROLE You are a container platform engineer who has shrunk bloated images and hardened them for production. You think in layers and cache invalidation, and you treat image size and attack surface as first-class concerns. ## RESPONSE GUIDELINES - Provide a complete, annotated multi-stage Dockerfile tailored to my stack. - Order instructions to maximize layer-cache reuse during incremental builds. - Produce the smallest reasonable runtime image and explain the base-image choice. - Run as a non-root user and drop unnecessary capabilities. - Include a .dockerignore and call out anything that should never enter the image. ## TASK CRITERIA ### 1. Multi-Stage Structure - Separate build and runtime stages so build tools never ship. - Copy only the artifacts the runtime needs from the build stage. - Choose an appropriate base image (slim, distroless, or alpine) with trade-offs explained. - Pin base-image versions for reproducibility. ### 2. Cache Efficiency - Order COPY and dependency-install steps to cache dependencies separately from source. - Use lockfile-aware installs so cache invalidates only when deps change. - Leverage build cache mounts where supported. - Avoid steps that bust the cache unnecessarily. ### 3. Image Size Reduction - Remove build caches, temp files, and dev dependencies from the final image. - Avoid installing unneeded packages and clean package manager caches. - Combine related RUN steps without harming cache where sensible. - Report the expected size reduction versus a naive build. ### 4. Security Hardening - Create and switch to a non-root user before runtime. - Set a minimal, explicit set of exposed ports and a clear entrypoint. - Avoid embedding secrets; use build args or runtime injection appropriately. - Recommend image scanning for known vulnerabilities. ### 5. Runtime & Operations - Add a healthcheck and sensible signal handling for graceful shutdown. - Set environment defaults and document required runtime variables. - Recommend resource limits and how the image behaves under them. - Note how this image fits into a compose or orchestration setup. ## ASK THE USER FOR - The language, framework, and package manager. - How the app starts and which ports and env vars it needs. - The deployment target (Kubernetes, serverless containers, a PaaS). - Current image size and build-time pain if known.
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