docker-debugger

# Docker Debugger ## Instructions When debugging Docker issues: 1. **Identify the problem type**: Build, runtime, networking, or performance 2. **Gather information** using diagnostic commands 3. **Analyze logs and errors** 4. **Apply fixes** ## Diagnostic Commands ```bash # Check running containers docker ps -a # View container logs docker logs <container_id> --tail 100 -f # Inspect container docker inspect <container_id> # Check resource usage docker stats # View container processes docker top <container_id> # Execute shell in running container docker exec -it <container_id> /bin/sh # Check Docker disk usage docker system df # View build history docker history <image_name> ``` ## Common Issues & Solutions ### 1. Container exits immediately ```bash # Check exit code docker inspect <container_id> --format='{{.State.ExitCode}}' # View last logs docker logs <container_id> ``` **Fixes**: - Ensure CMD/ENTRYPOINT runs a foreground process - Check for missing dependencies - Verify environment variables ### 2. Build fails ```dockerfile # Use multi-stage builds for smaller images FROM node:20-alpine AS builder WORKDIR /app COPY package*.json ./ RUN npm ci COPY . . RUN npm run build FROM node:20-alpine AS runner WORKDIR /app COPY --from=builder /app/dist ./dist COPY --from=builder /app/node_modules ./node_modules CMD ["node", "dist/index.js"] ``` ### 3. Networking issues ```bash # List networks docker network ls # Inspect network docker network inspect <network_name> # Check container IP docker inspect -f '{{range.NetworkSettings.Networks}}{{.IPAddress}}{{end}}' <container> ``` ### 4. Volume permission issues ```dockerfile # Create non-root user RUN addgroup -g 1001 -S appgroup && \ adduser -u 1001 -S appuser -G appgroup # Set ownership COPY --chown=appuser:appgroup . . USER appuser ``` ## Dockerfile Best Practices ```dockerfile # 1. Use specific tags, not :latest FROM node:20.10-alpine # 2. Set working directory WORKDIR /app # 3. Copy dependency files first (better caching) COPY package*.json ./ RUN npm ci --only=production # 4. Copy source after dependencies COPY . . # 5. Use non-root user USER node # 6. Set proper labels LABEL maintainer="you@example.com" LABEL version="1.0" # 7. Use HEALTHCHECK HEALTHCHECK --interval=30s --timeout=3s \ CMD wget -q --spider http://localhost:3000/health || exit 1 # 8. Expose ports EXPOSE 3000 # 9. Use exec form for CMD CMD ["node", "server.js"] ``` ## docker-compose Debugging ```yaml # docker-compose.yml services: app: build: context: . dockerfile: Dockerfile # Add for debugging stdin_open: true tty: true # Override command for debugging command: /bin/sh volumes: - .:/app environment: - DEBUG=true ``` ```bash # Rebuild without cache docker-compose build --no-cache # View logs docker-compose logs -f app # Restart single service docker-compose restart app ```