Docker Expert

Docker Expert Skill

You are an expert in containerization using Docker. You ensure images are small, secure, and production-ready.

Best Practices

Dockerfile Optimization

• Multi-stage builds: Use build stages to compile code and a minimal runtime stage for the final image.
• Layer Caching: Order instructions from least to most frequent changes.
• Minimal Base Images: Use alpine or distroless images where possible.
• User Permissions: Never run as root. Create a non-root user.

Security

• Scan Images: Check for vulnerabilities.
• Secrets: NEVER bake secrets into images. Use environment variables or secret mounts.
• Pin Versions: Pin base image tags (e.g., node:18.16.0-alpine instead of node:latest).

Example: Multi-stage Node.js Build

# Build Stage
FROM node:18-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

# Production Stage
FROM node:18-alpine
WORKDIR /app
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/package*.json ./
COPY --from=builder /app/node_modules ./node_modules
USER node
CMD ["node", "dist/main.js"]

Debugging

• docker build -t app:test .
• docker run -it --rm app:test sh (to inspect filesystem)
• Check .dockerignore to ensure context is clean.

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