Docker Composer Skill

Overview

This skill helps you create efficient Docker configurations for development and production. Covers Dockerfiles, Docker Compose, multi-stage builds, networking, volumes, and container orchestration best practices.

Docker Philosophy

Container Principles

1. One process per container: Keep containers focused
2. Immutable infrastructure: Don't modify running containers
3. Stateless containers: Store state in volumes or external services
4. Minimal images: Smaller = faster + more secure

Best Practices

• DO: Use multi-stage builds for production
• DO: Pin specific versions for dependencies
• DO: Use .dockerignore to exclude unnecessary files
• DO: Run as non-root user
• DON'T: Store secrets in images or Dockerfiles
• DON'T: Use latest tag in production
• DON'T: Install unnecessary packages

Dockerfile Patterns

Node.js Production Dockerfile

# Dockerfile
# ============================================
# Stage 1: Dependencies
# ============================================
FROM node:20-alpine AS deps
WORKDIR /app

# Install dependencies only when needed
COPY package.json package-lock.json ./
RUN npm ci --only=production

# ============================================
# Stage 2: Builder
# ============================================
FROM node:20-alpine AS builder
WORKDIR /app

COPY package.json package-lock.json ./
RUN npm ci

COPY . .

# Build application
ENV NEXT_TELEMETRY_DISABLED 1
RUN npm run build

# ============================================
# Stage 3: Runner (Production)
# ============================================
FROM node:20-alpine AS runner
WORKDIR /app

ENV NODE_ENV production
ENV NEXT_TELEMETRY_DISABLED 1

# Create non-root user
RUN addgroup --system --gid 1001 nodejs
RUN adduser --system --uid 1001 nextjs

# Copy built assets
COPY --from=builder /app/public ./public
COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static

USER nextjs

EXPOSE 3000

ENV PORT 3000
ENV HOSTNAME "0.0.0.0"

CMD ["node", "server.js"]

Python Production Dockerfile

# Dockerfile
# ============================================
# Stage 1: Builder
# ============================================
FROM python:3.11-slim AS builder

WORKDIR /app

# Install build dependencies
RUN apt-get update && apt-get install -y --no-install-recommends \
    build-essential \
    && rm -rf /var/lib/apt/lists/*

# Create virtual environment
RUN python -m venv /opt/venv
ENV PATH="/opt/venv/bin:$PATH"

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

# ============================================
# Stage 2: Runner
# ============================================
FROM python:3.11-slim AS runner

WORKDIR /app

# Create non-root user
RUN groupadd --gid 1000 appgroup \
    && useradd --uid 1000 --gid appgroup --shell /bin/bash appuser

# Copy virtual environment
COPY --from=builder /opt/venv /opt/venv
ENV PATH="/opt/venv/bin:$PATH"

# Copy application
COPY --chown=appuser:appgroup . .

USER appuser

EXPOSE 8000

CMD ["gunicorn", "--bind", "0.0.0.0:8000", "app:app"]

Development Dockerfile

# Dockerfile.dev
FROM node:20-alpine

WORKDIR /app

# Install development dependencies
RUN apk add --no-cache git

# Copy package files first (for caching)
COPY package.json package-lock.json ./

# Install all dependencies (including devDependencies)
RUN npm install

# Don't copy files - mount as volume for hot reload
# COPY . .

EXPOSE 3000

CMD ["npm", "run", "dev"]

Docker Compose Configurations

Full-Stack Development

# docker-compose.yml
version: '3.8'

services:
  # ===================
  # Application
  # ===================
  app:
    build:
      context: .
      dockerfile: Dockerfile.dev
    ports:
      - "3000:3000"
    volumes:
      - .:/app
      - /app/node_modules  # Exclude node_modules
    environment:
      - NODE_ENV=development
      - DATABASE_URL=postgresql://postgres:postgres@db:5432/myapp
      - REDIS_URL=redis://redis:6379
    depends_on:
      db:
        condition: service_healthy
      redis:
        condition: service_started
    networks:
      - app-network

  # ===================
  # Database
  # ===================
  db:
    image: postgres:15-alpine
    ports:
      - "5432:5432"
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: postgres
      POSTGRES_DB: myapp
    volumes:
      - postgres_data:/var/lib/postgresql/data
      - ./init.sql:/docker-entrypoint-initdb.d/init.sql
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 5s
      timeout: 5s
      retries: 5
    networks:
      - app-network

  # ===================
  # Redis Cache
  # ===================
  redis:
    image: redis:7-alpine
    ports:
      - "6379:6379"
    volumes:
      - redis_data:/data
  

...