Docker Helper Skill

Docker Compose generation, optimization, and troubleshooting assistance.

Instructions

You are a Docker and containerization expert. When invoked:

1. Generate Docker Files:

   • Create Dockerfile based on project type
   • Generate docker-compose.yml for multi-service apps
   • Optimize for build time and image size
   • Follow best practices for security and performance

2. Optimize Existing Configurations:

   • Reduce image sizes (multi-stage builds)
   • Improve layer caching
   • Security hardening
   • Resource limits and health checks

3. Troubleshoot Issues:

   • Container startup failures
   • Network connectivity problems
   • Volume mounting issues
   • Performance problems

4. Provide Best Practices:

   • Image naming and tagging
   • Secrets management
   • Logging configuration
   • Development vs production configs

Dockerfile Best Practices

Node.js Application

# Multi-stage build for smaller image
FROM node:18-alpine AS builder

WORKDIR /app

# Copy package files first (better layer caching)
COPY package*.json ./
RUN npm ci --only=production

# Copy application code
COPY . .
RUN npm run build

# Production stage
FROM node:18-alpine

WORKDIR /app

# Run as non-root user
RUN addgroup -g 1001 -S nodejs && \
    adduser -S nodejs -u 1001

# Copy only necessary files from builder
COPY --from=builder --chown=nodejs:nodejs /app/dist ./dist
COPY --from=builder --chown=nodejs:nodejs /app/node_modules ./node_modules
COPY --chown=nodejs:nodejs package*.json ./

USER nodejs

EXPOSE 3000

# Use exec form for proper signal handling
CMD ["node", "dist/index.js"]

# Health check
HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
  CMD node healthcheck.js

Python Application

FROM python:3.11-slim

WORKDIR /app

# Install dependencies in separate layer
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

# Copy application
COPY . .

# Create non-root user
RUN useradd -m -u 1001 appuser && \
    chown -R appuser:appuser /app

USER appuser

EXPOSE 8000

CMD ["python", "app.py"]

Go Application

# Build stage
FROM golang:1.21-alpine AS builder

WORKDIR /app

COPY go.mod go.sum ./
RUN go mod download

COPY . .
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main .

# Final stage - minimal image
FROM alpine:latest

RUN apk --no-cache add ca-certificates

WORKDIR /root/

COPY --from=builder /app/main .

EXPOSE 8080

CMD ["./main"]

Docker Compose Examples

Full Stack Application

version: '3.8'

services:
  # Frontend
  web:
    build:
      context: ./web
      dockerfile: Dockerfile
    ports:
      - "3000:3000"
    environment:
      - NODE_ENV=production
      - API_URL=http://api:8000
    depends_on:
      api:
        condition: service_healthy
    networks:
      - frontend
    restart: unless-stopped

  # Backend API
  api:
    build:
      context: ./api
      dockerfile: Dockerfile
    ports:
      - "8000:8000"
    environment:
      - DATABASE_URL=postgresql://user:password@db:5432/myapp
      - REDIS_URL=redis://cache:6379
    env_file:
      - .env
    depends_on:
      db:
        condition: service_healthy
      cache:
        condition: service_started
    networks:
      - frontend
      - backend
    volumes:
      - ./logs:/app/logs
    restart: unless-stopped
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:8000/health"]
      interval: 30s
      timeout: 10s
      retries: 3
      start_period: 40s

  # Database
  db:
    image: postgres:15-alpine
    environment:
      - POSTGRES_DB=myapp
      - POSTGRES_USER=user
      - POSTGRES_PASSWORD=password
    volumes:
      - postgres_data:/var/lib/postgresql/data
      - ./init.sql:/docker-entrypoint-initdb.d/init.sql:ro
    networks:
      - backend
    restart: unless-stopped
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U user -d myapp"]
      interval: 10s
      timeout: 5s
      retries: 5

  # Redis Cache
  cache:
    image: redis:7-alpine
    command: redis-server --appendonly yes
    volumes:
      - redis_data:/data
    networks:
      - backend
    restart: unless-stopped

  # Nginx Reverse Proxy
  nginx:
    image: nginx:alpine
    ports:
      - "80:80"
      - "443:443"
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf:ro
      - ./ssl:/etc/nginx/ssl:ro
    depends_on:
      - web
      - api
    networks:
      - frontend
    restart: unless-stopped

networks:
  frontend:
    driver: bridge
  backend:
    driver: bridge

volumes:
  postgres_data:
  redis_data:

Development Environment

version: '3.8'

services:
  app:
    build:
      context: .
      dockerfile: Dockerfile.dev
    ports:
      - "3000:3000"
    environment:
      - NODE_ENV=development
    volumes:
      - .:/app
      - /app/node_modules  # Anonymous volume for node_modules
    command: npm run dev
    networks:
      - dev_network

  db:
    image: postgres:15-alpine
    environment:
      - POSTGRES_DB=dev_db
      - POSTGRES_USER=dev
      - POSTGRES_PASSWORD=dev_password
    ports:
      - "5432:5432"
    volumes:
      - dev_db_data:/var/lib/postgresql/data
    networks:
      - dev_network

volumes:
  dev_db_data:

networks:
  dev_network:

Usage Examples

@docker-helper
@docker-helper --generate-dockerfile
@docker-helper --optimize
@docker-helper --compose
@docker-helper --troubleshoot

Optimization Techniques

Multi-Stage Builds

# Reduces final image size by 70-90%
FROM node:18 AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

FROM node:18-alpine
WORKDIR /app
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules
CMD ["node", "dist/index.js"]

Layer Caching

# ❌ Bad - Invalidates cache on any file change
COPY . .
RUN npm install

# ✓ Good - Cache dependencies separately
COPY package*.json ./
RUN npm install
COPY . .

Reduce Image Size

# Use alpine variants (much smaller)
FROM node:18-alpine  # ~170MB vs ~900MB for node:18

# Clean up in same layer
RUN apt-get update && \
    apt-get install -y package && \
    apt-get clean && \
    rm -rf /var/lib/apt/lists/*

# Use .dockerignore
# Create .dockerignore file:
# node_modules
# .git
# *.md
# .env*

Security Best Practices

# Don't run as root
RUN adduser -D -u 1001 appuser
USER appuser

# Scan for vulnerabilities
# Use: docker scan myimage:tag

# Use specific tags, not 'latest'
FROM node:18.16.0-alpine  # Not: FROM node:latest

# Don't store secrets in image
# Use environment variables or secrets management

# Minimize attack surface
# Use minimal base images (alpine, distroless)

# Keep base images updated
# Regularly rebuild and update

Common Issues & Solutions

Issue: Container Exits Immediately

# Check logs
docker logs <container_id>

# Run interactively to debug
docker run -it <image> /bin/sh

# Check entrypoint/command
docker inspect <container_id> | grep -A5 Cmd

Issue: Cannot Connect to Service

# Ensure services are on same network
networks:
  - mynetwork

# Use service name as hostname
DATABASE_URL=postgresql://db:5432/myapp  # 'db' is service name

# Check if service is ready
depends_on:
  db:
    condition: service_healthy

Issue: Volume Permission Problems

# Match host user ID
RUN adduser -u 1001 appuser
USER appuser

# Or change ownership in entrypoint
ENTRYPOINT ["sh", "-c", "chown -R appuser:appuser /data && exec \"$@\""]

Issue: Slow Builds

# Use build cache effectively
COPY package*.json ./
RUN npm ci
COPY . .

# Use BuildKit
# Set: DOCKER_BUILDKIT=1

# Use .dockerignore
# Exclude: node_modules, .git, build artifacts

Docker Commands Reference

# Build image
docker build -t myapp:latest .

# Run container
docker run -d -p 3000:3000 --name myapp myapp:latest

# View logs
docker logs -f myapp

# Execute command in container
docker exec -it myapp /bin/sh

# Stop and remove
docker stop myapp && docker rm myapp

# Compose commands
docker-compose up -d
docker-compose down
docker-compose logs -f
docker-compose ps

# Clean up
docker system prune -a
docker volume prune

Health Checks

# Node.js
HEALTHCHECK --interval=30s --timeout=3s \
  CMD node healthcheck.js || exit 1

# Python
HEALTHCHECK --interval=30s --timeout=3s \
  CMD python -c "import requests; requests.get('http://localhost:8000/health')" || exit 1

# Simple HTTP check
HEALTHCHECK --interval=30s --timeout=3s \
  CMD wget --no-verbose --tries=1 --spider http://localhost:8000/health || exit 1

Notes

• Always use specific version tags, not latest
• Implement health checks for critical services
• Use multi-stage builds to reduce image size
• Never store secrets in Dockerfiles or images
• Use .dockerignore to exclude unnecessary files
• Run containers as non-root users
• Implement proper logging (stdout/stderr)
• Use volumes for persistent data
• Configure resource limits in production
• Regularly update base images for security patches