Images, containers and your first Dockerfile

Learn why Docker: 'it works on my machine' becomes 'it works everywhere'. An image is a packaged app (code + Python + libraries); a container is a running image. Write a Dockerfile for the API: slim Python base, copy requirements first (so the install step is cached), copy code, run as a non-root user, expose the port, start Uvicorn. Build and run it locally.