Image, Container, Registry

The Three Words That Explain Docker

Term	What it is	Analogy
Image	Read-only template: filesystem + metadata	A class / a recipe
Container	A running (or stopped) instance of an image	An object / a baked cake
Registry	Server that stores and distributes images	GitHub, but for images

One image can run as many containers - each gets its own thin writable layer on top of the shared read-only image.

$ docker pull nginx:1.27-alpine     # registry  →  local image
$ docker run  nginx:1.27-alpine     # image     →  running container
$ docker push myrepo/myapp:v1       # local image → registry

Image names decode as registry/repository:tag:

nginx:1.27-alpine → Docker Hub (default registry), repo nginx, tag 1.27-alpine
ghcr.io/org/api:v2 → GitHub's registry, repo org/api, tag v2

Warning: The tag latest is just a naming convention - it is not guaranteed to be the newest version, and it changes underneath you. Production deployments pin exact tags (or digests), never latest.

Goal: Ready to run your first container? Head to the Run Your First Container lab in the Docker track - you get a real Docker daemon and a live browser preview of what you deploy.