What is Docker and why do developers use it?

Docker is a containerisation platform that packages your application and all its dependencies into a portable container.

**Why use it:**
- **Consistency** — works the same on dev, staging, and production
- **Isolation** — each app runs in its own environment
- **Fast** — containers start in milliseconds
- **Lightweight** — shares the host OS kernel (unlike VMs)

```bash
docker build -t myapp .
docker run -p 3000:3000 myapp
```