Skillversitas

Docker Containers

A Docker container is a runtime instance of a Docker image. A container runs as an isolated process on the host.

Container Lifecycle

Created → Running → Paused → Stopped → Removed
   ↑         ↓        ↓         ↓
   └─────────┴────────┴─────────┘

Basic Container Commands

# Run a new container
docker run -d --name mycontainer nginx

# View running containers
docker ps

# View all containers (including stopped ones)
docker ps -a

# Stop a container
docker stop mycontainer

# Start a stopped container
docker start mycontainer

# Restart a container
docker restart mycontainer

# Delete a container
docker rm mycontainer

Running Containers

# Run with port mapping
docker run -d -p 8080:80 --name webserver nginx

# Run with a volume
docker run -d -v /host/path:/container/path --name data nginx

# Run with an environment variable
docker run -d -e POSTGRES_PASSWORD=secret --name db postgres

# Run interactively
docker run -it ubuntu bash

Accessing Containers

# Enter a running container
docker exec -it mycontainer bash

# Run a command inside a container
docker exec mycontainer ls -la

# View a container's logs
docker logs mycontainer

# View logs in real time
docker logs -f mycontainer

Resource Management

# Limit CPU
docker run -d --cpus="1.5" myapp:latest

# Limit memory
docker run -d --memory="512m" myapp:latest

# View resource usage
docker stats

Container Networking

# Run with a specific port
docker run -d -p 8080:80 nginx

# Run on a specific network
docker run -d --network mynetwork --name api myapp

# View port mapping
docker port mycontainer

Use Case: Application Containers

# Run a PostgreSQL database
docker run -d \
  --name myapp-db \
  -e POSTGRES_DB=myapp \
  -e POSTGRES_USER=admin \
  -e POSTGRES_PASSWORD=secret123 \
  -v myapp-db-data:/var/lib/postgresql/data \
  -p 5432:5432 \
  postgres:15

# Run the backend application
docker run -d \
  --name myapp-api \
  --link myapp-db:database \
  -e DB_HOST=database \
  -e DB_PORT=5432 \
  -e DB_NAME=myapp \
  -e DB_USER=admin \
  -e DB_PASSWORD=secret123 \
  -p 3000:3000 \
  myapp-api:1.0

# Run the frontend
docker run -d \
  --name myapp-frontend \
  --link myapp-api:api \
  -e API_URL=http://api:3000 \
  -p 8080:80 \
  myapp-frontend:1.0

Monitoring Containers

# View real-time stats
docker stats

# View processes inside a container
docker top mycontainer

# View container inspect data
docker inspect mycontainer

# View disk usage
docker system df

Everyday Use Case

Case: A developer needs to run the application for testing.

# Run the application with a database
docker run -d \
  --name test-app \
  -p 3000:3000 \
  -e NODE_ENV=development \
  -v $(pwd):/app \
  -w /app \
  node:18-alpine \
  npm run dev

# Access the application
curl http://localhost:3000

# View logs
docker logs -f test-app

Cleanup

# Delete stopped containers
docker container prune

# Delete all containers
docker rm -f $(docker ps -aq)

# Delete everything (containers, images, volumes)
docker system prune -a --volumes

Best Practices

  1. Use --name: Give descriptive names
  2. Restart policy: Use --restart unless-stopped for production
  3. Health checks: Define a health check for monitoring
  4. Resource limits: Set CPU and memory limits
  5. Don't run as root: Use --user when possible

Use Case: Health Check

# Dockerfile
FROM node:18-alpine
HEALTHCHECK --interval=30s --timeout=3s \
  CMD curl -f http://localhost:3000/health || exit 1
CMD ["node", "server.js"]

# Run with a health check
docker run -d \
  --name myapp-api \
  --health-cmd="curl -f http://localhost:3000/health || exit 1" \
  --health-interval=30s \
  --health-timeout=3s \
  -p 3000:3000 \
  myapp-api:1.0

Conclusion

Containers are the basic unit in Docker. Understanding how to run, manage, and monitor containers is essential for managing applications that run in containers.