01 — Docker Fundamentals

What is Docker?

Docker packages applications and their dependencies into containers — lightweight, portable, isolated environments that run consistently everywhere.

Traditional:                    Docker:
App → depends on OS libs       App + dependencies → Container
"Works on my machine" 😢       "Works everywhere" ✅

---

Containers vs VMs

VM:                          Container:
┌─────────┐ ┌─────────┐    ┌─────┐ ┌─────┐ ┌─────┐
│  App A  │ │  App B  │    │App A│ │App B│ │App C│
│  Libs   │ │  Libs   │    │Libs │ │Libs │ │Libs │
│ Guest OS│ │ Guest OS│    └──┬──┘ └──┬──┘ └──┬──┘
└────┬────┘ └────┬────┘       │      │      │
     │           │         ┌──┴──────┴──────┴──┐
┌────┴───────────┴────┐    │   Docker Engine    │
│    Hypervisor       │    └────────┬───────────┘
└─────────┬───────────┘             │
     Host OS                   Host OS

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Docker Architecture

┌──────────────────────────────────────────────┐
│                Docker Client (CLI)            │
│  docker build, docker run, docker pull       │
└─────────────────────┬────────────────────────┘
                      │ REST API
┌─────────────────────┴────────────────────────┐
│              Docker Daemon (dockerd)          │
│                                               │
│  ┌───────────┐  ┌───────────┐  ┌───────────┐│
│  │  Images   │  │Containers │  │  Volumes   ││
│  └───────────┘  └───────────┘  └───────────┘│
│  ┌───────────┐                               │
│  │ Networks  │                               │
│  └───────────┘                               │
└──────────────────────────────────────────────┘

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Essential Commands

# Images
docker pull nginx                    # Download image
docker images                        # List images
docker rmi nginx                     # Remove image
docker build -t my-app .             # Build from Dockerfile

# Containers
docker run -d -p 3000:3000 my-app   # Run detached, map port
docker ps                            # List running containers
docker ps -a                         # List all (including stopped)
docker stop <id>                     # Stop container
docker rm <id>                       # Remove container
docker logs <id>                     # View logs
docker exec -it <id> sh             # Shell into container

# Cleanup
docker system prune                  # Remove unused data
docker system prune -a               # Remove all unused images too

---

Running Your First Container

# Run Nginx web server
docker run -d -p 8080:80 --name web nginx
# Visit http://localhost:8080

# Run Node.js app
docker run -d -p 3000:3000 --name api \
  -e NODE_ENV=production \
  -e DATABASE_URL=postgres://... \
  my-node-app

# Run PostgreSQL
docker run -d -p 5432:5432 --name db \
  -e POSTGRES_PASSWORD=secret \
  -e POSTGRES_DB=myapp \
  -v pgdata:/var/lib/postgresql/data \
  postgres:16

# Run Redis
docker run -d -p 6379:6379 --name cache redis:7-alpine

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Key Takeaways

• Docker containers are lightweight, portable, and start in seconds
• Images are immutable blueprints; containers are running instances
• Use -d for detached mode, -p for port mapping, -e for environment variables
• Use -v for persistent volumes (data survives container restarts)
• docker exec -it <id> sh to debug inside a running container
• docker system prune to reclaim disk space