I thought that tags in Docker worked like in stackoverflow where millions of questions can be tagged with the same tag. But when I tag a second image in Docker the first one loses its tag:

So are images to tags one-to-many, i.e. one image can have multiple tags in a repo, but a tag cannot be applied to 2 or more images in the same repo?

CodePudding user response：

Pushing a new tag replaces the old tag, but if you know the digest, you can pull the old manifest until the registry garbage collects it.

A tag is a pointer to a manifest in the registry, and it can only point to a single manifest, similar to a symlink in Linux. This is needed since everything else in the registry is content addressable, so you need the tag to avoid needing to remember long digests.

There are a couple manifest types, an image manifest, and a manifest list. The manifest list contains references to other manifests, which is commonly used for multi-platform images. So a tag pointing to a manifest list could refer to multiple images using a manifest list. But runtimes will only pull a single image out of that list. And that list is generated by the tool pushing the image, not dynamically created by the registry by merging the previous images into a list (that would break the content addressable logic since it would change the digest).