Tags, push, and pull

Registries

So far your image exists only on the machine that built it, which means nothing else can run it: not the server you deploy to, not a teammate, not the test automation of unit 7. Moving images between machines is what a registry is for: a server that stores and serves images, git-hosting for images instead of code. You already used one: docker run hello-world in lesson 2-1 pulled from Docker Hub, the default public registry. The other one you will meet constantly is GitHub Container Registry (GHCR), which lives next to your repos.

A full image reference has up to four parts:

ghcr.io/ada/shop:v2
└─────┘ └─┘ └──┘ └┘
registry owner name tag

When you write just nginx, Docker fills in the defaults: docker.io/library/nginx:latest.

Shipping an image is three commands:

docker login ghcr.io                      # authenticate once
docker tag myapp:v1 ghcr.io/ada/shop:v1   # give the image its full remote name
docker push ghcr.io/ada/shop:v1           # upload it

And any other machine runs docker pull ghcr.io/ada/shop:v1. Thanks to layers (lesson 3-2), pushes and pulls only transfer layers the other side doesn't already have.