# Docker for Developers Docker enables you to package applications with all their dependencies into standardized units for development, testing, and deployment. ## Core Concepts ### Images and Containers An **image** is a read-only template with instructions for creating a Docker container. A **container** is a runnable instance of an image. ### Dockerfile ```dockerfile FROM node:20-alpine WORKDIR /app COPY package*.json ./ RUN npm ci --only=production COPY . . EXPOSE 3000 CMD ["node", "server.js"] ``` ## Docker Compose For multi-service applications: ```yaml services: web: build: . ports: - "3000:3000" depends_on: - db db: image: postgres:15 environment: POSTGRES_DB: myapp ``` ## Best Practices 1. Use multi-stage builds to reduce image size 2. Leverage build cache with proper layer ordering 3. Use `.dockerignore` to exclude unnecessary files 4. Run containers as non-root users