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

app/models/container_image.rb

+class ContainerImage < ActiveRecord::Base
+  belongs_to :container_images_repository
+
+  delegate :registry, :registry_path_with_namespace, :client, to: :container_images_repository
+
+  validates :manifest, presence: true
+
+  before_validation :update_token, on: :create
+  def update_token
+    paths = container_images_repository.allowed_paths << name_with_namespace
+    token = Auth::ContainerRegistryAuthenticationService.full_access_token(paths)
+    client.update_token(token)
+  end
+
+  def path
+    [registry.path, name_with_namespace].compact.join('/')
+  end
+
+  def name_with_namespace
+    [registry_path_with_namespace, name].compact.join('/')
+  end
+
+  def tag(tag)
+    ContainerRegistry::Tag.new(self, tag)
+  end
+
+  def manifest
+    @manifest ||= client.repository_tags(name_with_namespace)
+  end
+
+  def tags
+    return @tags if defined?(@tags)
+    return [] unless manifest && manifest['tags']
+
+    @tags = manifest['tags'].map do |tag|
+      ContainerRegistry::Tag.new(self, tag)
+    end
+  end
+
+  def blob(config)
+    ContainerRegistry::Blob.new(self, config)
+  end
+
+  def delete_tags
+    return unless tags
+
+    tags.all?(&:delete)
+  end
+
+  def self.split_namespace(full_path)
+    image_name = full_path.split('/').last
+    namespace = full_path.gsub(/(.*)(#{Regexp.escape('/' + image_name)})/, '\1')
+    if namespace.count('/') < 1
+      namespace, image_name = full_path, ""
+    end
+    return namespace, image_name
+  end
+end