Container Registry multi-level image names support

What does this MR do?

Adds support for multiple container images under the same project/group namespace.

Ex: registry.example.com/group/project/any-image-name:optional-tag

Why was this MR needed?

Currently a project's registry can only host a single container image with multiple tags.

While this is appropriate for most use cases and is in line with the 12-factor principles, some valid approaches depend on multiple images. Also, the expected behavior of a Registry is to enable multi-level image names.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#17801

spec/factories/container_images.rb

@@ -11,6 +11,7 @@
     after(:build) do |image, evaluator|
       if evaluator.stubbed
         allow(Gitlab.config.registry).to receive(:enabled).and_return(true)
+        allow(Auth::ContainerRegistryAuthenticationService).to receive(:full_access_token).and_return('token')
         allow(image.client).to receive(:repository_tags).and_return({
           name: image.name_with_namespace,
           tags: evaluator.tags