Multi Stage Dockerfile Agent Skill

Your goal is to help me create efficient multi-stage Dockerfiles that follow best practices, resulting in smaller, more secure container images.

Multi-Stage Structure

• Use a builder stage for compilation, dependency installation, and other build-time operations
• Use a separate runtime stage that only includes what's needed to run the application
• Copy only the necessary artifacts from the builder stage to the runtime stage
• Use meaningful stage names with the AS keyword (e.g., FROM node:18 AS builder)
• Place stages in logical order: dependencies → build → test → runtime

Base Images

• Start with official, minimal base images when possible
• Specify exact version tags to ensure reproducible builds (e.g., python:3.11-slim not just python)
• Consider distroless images for runtime stages where appropriate
• Use Alpine-based images for smaller footprints when compatible with your application
• Ensure the runtime image has the minimal necessary dependencies

Layer Optimization

• Organize commands to maximize layer caching
• Place commands that change frequently (like code changes) after commands that change less frequently (like dependency installation)
• Use .dockerignore to prevent unnecessary files from being included in the build context
• Combine related RUN commands with && to reduce layer count
• Consider using COPY --chown to set permissions in one step

Security Practices

• Avoid running containers as root - use USER instruction to specify a non-root user
• Remove build tools and unnecessary packages from the final image
• Scan the final image for vulnerabilities
• Set restrictive file permissions
• Use multi-stage builds to avoid including build secrets in the final image

Performance Considerations

• Use build arguments for configuration that might change between environments
• Leverage build cache efficiently by ordering layers from least to most frequently changing
• Consider parallelization in build steps when possible
• Set appropriate environment variables like NODE_ENV=production to optimize runtime behavior
• Use appropriate healthchecks for the application type with the HEALTHCHECK instruction