dockerfile-reviewer

Description

Review Dockerfiles and docker-compose files for security vulnerabilities, oversized images, build inefficiencies, and missing best practices. Returns a structured report with severity ratings and corrected examples.

Use when

• "review my Dockerfile"
• "is this container secure"
• "optimize my docker build"
• "why is my image so large"
• "check my docker-compose"
• Any Dockerfile, docker-compose.yml, or .dockerignore

Input

Paste the Dockerfile and/or docker-compose.yml. Optionally specify:

• Target environment (production, CI, local dev)
• Base image constraints (must use specific distro, etc.)
• Whether the app runs as a service or a one-shot job

Output format

## Dockerfile Review

### Critical (fix before production)
- [Finding] — [security or correctness risk]
  ✗ Before: [problematic line(s)]
  ✓ After:  [corrected line(s)]

### Warnings (should fix)
- [Finding] — [size or reliability impact]

### Suggestions (nice to have)
- [Finding] — [explanation]

### What's correct
- [Specific patterns done right]

### Summary
[2–3 sentences: biggest risk, estimated image size savings if any, top fix]

Review checklist

Security

• Running as root (no USER directive) — container escape risk
• Secret or credential in ENV, ARG, or RUN layer — visible in image history
• Base image not pinned (FROM ubuntu:latest instead of ubuntu:22.04) — supply chain risk
• Using curl | bash to install software — arbitrary code execution
• Unnecessary packages installed (attack surface)
• No HEALTHCHECK — orchestrator can't detect unhealthy containers
• Writable filesystem where read-only would suffice

Image size

• Large base image when alpine or distroless would work
• Installing dev tools in production image (compilers, debuggers, test frameworks)
• Multiple RUN commands that should be chained with && (each RUN = a layer)
• COPY . . before dependency install (cache busting on every code change)
• Not using .dockerignore — copying node_modules, .git, build artifacts
• Leftover apt/apk cache not cleaned in same RUN layer

Build correctness

• Wrong WORKDIR — files land in unexpected paths
• EXPOSE port doesn't match what the app actually listens on
• CMD vs ENTRYPOINT confusion — CMD should be overridable args, ENTRYPOINT the executable
• Using ADD when COPY is sufficient (ADD has implicit tar extraction and URL fetch)
• Build args used as secrets (visible in docker history)

docker-compose specific

• No restart policy — containers don't recover from crashes
• Hardcoded secrets in environment: block — use .env or secrets
• Named volumes not defined in volumes: section
• Port binding to 0.0.0.0 when 127.0.0.1 would suffice
• No resource limits (mem_limit, cpus) — one container can starve others
• Depends_on without condition: service_healthy — race conditions on startup

Multi-stage build

• Single-stage build for compiled language — ships compiler in production image
• Build artifacts not properly copied from builder stage
• Redundant stages that could be merged

Severity definitions

• Critical: Security vulnerability or correctness bug that affects production
• Warning: Image bloat, reliability issue, or hard-to-debug behavior
• Suggestion: Style, caching efficiency, or future-proofing improvement

Self-improvement instructions

After each review, note the most impactful finding. After 20 reviews, surface "Top 3 Dockerfile mistakes" at the start of the response.