Image Names and Tags

You type docker pull postgres and something downloads. From where? Which version? A teammate's laptop pulled the "same" image last month and got a different one. Every image name hides several parts, and Docker quietly fills in the ones you leave out.

The rule

Where / who / what : which version — and Docker fills the blanks.

A full image reference has four parts:

• Registry: the server that stores the image. If you leave it out, Docker uses Docker Hub, docker.io.
• Namespace: the account or organisation that owns it. On Docker Hub, official images live under library, which you never type.
• Repository: the image's name.
• Tag: a label for one version. If you leave it out, Docker uses latest.

So these pairs are identical:

Two commands help you see what you have:

docker images lists each local image with its repository, tag, ID and size.

Two warnings about tags. First, latest is only a default name, not a promise of "newest"; its owner can point it at a different image at any time. Second, even a tag like 16 moves as patch releases come out. For exact repeatability you can pin a digest, the content hash of one precise image: postgres@sha256:….

The shortcut

Expand before you trust. Whenever you read a short name, mentally fill in the missing parts, left to right: registry? namespace? tag?

Drills

Answer fast — trust the rule.

Remember

• A full name is registry / namespace / repository : tag; Docker fills in docker.io, library and latest.
• Shortcut: expand before you trust, left to right.
• Trap: latest means "default", not "newest", and tags can move; digests cannot.