Dockerfile Generation Skill

Generate production-ready Dockerfiles through iterative verification.

Scope Constraints

• Read access to codebase for analysis; write access limited to Dockerfile and .dockerignore
• Executes docker build and docker run for verification only — no deployment or registry push
• Does not modify application source code or dependency files
• Does not configure CI/CD pipelines — delegates to cicd-generation skill

Input Sanitization

• Base image names: alphanumeric, hyphens, dots, slashes, colons only — reject shell metacharacters and null bytes
• Port numbers: integers 1-65535 only
• File paths in COPY directives: reject .. traversal and null bytes

Core Principles

1. Verification-first: Examine codebase BEFORE adding ANY instruction
2. Multi-arch: Support amd64/arm64, no hardcoded paths
3. No HEALTHCHECK: Cannot verify health endpoints from code alone
4. Multi-stage builds: Builder stage → minimal runtime stage
5. Security: Non-root user (UID 10001+), pinned versions, no :latest
6. Layer optimization: Dependencies first, source second

Process

Step 1: Analyze Codebase

Before writing ANY Dockerfile instruction, verify:

[ ] Language/runtime detected (package.json, requirements.txt, go.mod, etc.)
[ ] Entry point identified (main file, start script)
[ ] Build command identified (if compilation needed)
[ ] Dependencies file location confirmed
[ ] Required environment variables documented
[ ] Exposed ports identified from code (NOT assumed)

Step 2: Generate Dockerfile

Structure:

# syntax=docker/dockerfile:1

# Build stage
FROM <base>:<pinned-version> AS builder
WORKDIR /app
# Install deps first (cache layer)
COPY <deps-file> .
RUN <install-deps>
# Copy source
COPY . .
RUN <build-command>

# Runtime stage
FROM <minimal-base>:<pinned-version>
WORKDIR /app

# Security: non-root user
RUN addgroup --gid 10001 appgroup && \
    adduser --uid 10001 --ingroup appgroup --disabled-password appuser
USER appuser

# Copy artifacts from builder
COPY --from=builder --chown=appuser:appgroup /app/<artifact> .

EXPOSE <verified-port>
CMD ["<verified-entrypoint>"]

Language-specific bases: | Language | Builder | Runtime | |----------|---------|---------| | Node.js | node:<version>-alpine | node:<version>-alpine | | Python | python:<version>-slim | python:<version>-slim | | Go | golang:<version>-alpine | gcr.io/distroless/static | | Java | eclipse-temurin:<version> | eclipse-temurin:<version>-jre-alpine | | Rust | rust:<version>-alpine | gcr.io/distroless/cc |

Step 3: Validation Loop (Max 5 iterations)

# Build
docker build -t test-image:local .

# Run and capture logs
docker run --rm test-image:local 2>&1 | head -50

# If error: analyze → fix → rebuild
# If success: verify functionality

Stop conditions:

• Container starts and responds correctly
• 5 iterations reached (escalate to user)

Gotchas

• COPY . . before COPY package*.json && npm install busts the dependency cache on every source change — always copy deps files first
• Alpine-based images use musl not glibc — native Node.js addons (bcrypt, sharp) may fail with cryptic errors. Use -slim for compatibility
• RUN apt-get update && apt-get install without rm -rf /var/lib/apt/lists/* bloats the image by 30-100MB
• Multi-stage builds discard everything not explicitly COPY --from=builder — forgetting to copy a config file causes silent runtime failures
• EXPOSE is documentation only — it does NOT publish the port. You still need -p at runtime
• Running as root (default) means container compromise = full host access — always add a non-root user with UID 10001+

Anti-patterns to Avoid

• FROM <image>:latest - Always pin versions
• RUN apt-get update && apt-get install without cleanup
• COPY . . before dependency installation
• Running as root
• Hardcoded secrets or credentials
• Assuming ports without code verification
• Adding HEALTHCHECK without verified endpoint

Output Format

When generating a Dockerfile, output:

1. Analysis summary: What was detected in codebase
2. Dockerfile: The generated file
3. Build command: Exact command to build
4. Run command: Exact command to test
5. Notes: Any assumptions or manual verification needed