Docker Basics

A beginner-friendly guide to Docker containerization.

What is Docker?

Docker packages your application and its dependencies into a container - a lightweight, standalone, executable unit that runs consistently across any environment.

Key Concepts

  • Image: A blueprint for a container (like a class)
  • Container: A running instance of an image (like an object)
  • Dockerfile: Instructions to build an image
  • Docker Compose: Tool for defining multi-container apps

Getting Started

Install Docker

Download Docker Desktop from docker.com.

# Verify installation
docker --version
docker compose version

Your First Dockerfile

Create a Dockerfile in your project:

# Use Node.js base image
FROM node:20-alpine

# Set working directory
WORKDIR /app

# Copy package files
COPY package*.json ./

# Install dependencies
RUN npm install

# Copy source code
COPY . .

# Expose port
EXPOSE 3000

# Start the application
CMD ["npm", "start"]

Build and Run

# Build the image
docker build -t my-app .

# Run the container
docker run -p 3000:3000 my-app

Visit http://localhost:3000 to see your app!

Docker Compose

For multi-service applications, use docker-compose.yml:

version: '3.8'
services:
  app:
    build: .
    ports:
      - "3000:3000"
    volumes:
      - .:/app
    depends_on:
      - db

  db:
    image: postgres:15
    environment:
      POSTGRES_DB: mydb
      POSTGRES_PASSWORD: secret
    ports:
      - "5432:5432"

# Start all services
docker compose up -d

# Stop all services
docker compose down

# View logs
docker compose logs -f

Essential Commands

# Images
docker images              # List images
docker pull image:tag      # Download an image
docker rmi image:tag       # Remove an image

# Containers
docker ps                  # List running containers
docker ps -a               # List all containers
docker stop container_id   # Stop a container
docker rm container_id     # Remove a container

# Debugging
docker logs container_id   # View container logs
docker exec -it id bash    # Shell into container

Best Practices

  1. Use .dockerignore to exclude unnecessary files
  2. Multi-stage builds for smaller images
  3. Don't run as root in containers
  4. Use specific image tags (not latest)
  5. Layer caching - put rarely changing steps first
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