Update container repository path reference

We should allow to use double underscore in the path, and it seems that
our container repository path regexp was outdated.

See https://github.com/docker/distribution/blob/master/reference/regexp.go

lib/gitlab/regex.rb

@@ -25,7 +25,7 @@ module Gitlab
     # See https://github.com/docker/distribution/blob/master/reference/regexp.go.
     #
     def container_repository_name_regex
-      @container_repository_regex ||= %r{\A[a-z0-9]+(?:[-._/][a-z0-9]+)*\Z}
+      @container_repository_regex ||= %r{\A[a-z0-9]+((?:[._/]|__|[-])[a-z0-9]+)*\Z}
     end
 
     ##

spec/lib/container_registry/path_spec.rb

@@ -86,6 +86,24 @@ describe ContainerRegistry::Path do
 
       it { is_expected.to be_valid }
     end
+
+    context 'when path contains double underscore' do
+      let(:path) { 'my/repository__name' }
+
+      it { is_expected.to be_valid }
+    end
+
+    context 'when path contains invalid separator with dot' do
+      let(:path) { 'some/registry-.name' }
+
+      it { is_expected.not_to be_valid }
+    end
+
+    context 'when path contains invalid separator with underscore' do
+      let(:path) { 'some/registry._name' }
+
+      it { is_expected.not_to be_valid }
+    end
   end
 
   describe '#has_repository?' do