Handle auth error for container registry

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/348727

Modifies the auth service for container registry to return an error to satisfy https://gitlab.com/gitlab-org/gitlab/-/issues/348727

We take this approach because in Rails we can only handle this during JWT authentication, see the conversation here, and the summary here

How to set up and validate locally

1. Setup Container Registry in your local instance by following the docs
2. Modify this method to return true
3. Attempt to push an image, which should not succeed, e.g.

     docker push gdk.test:5000/gitlab-org/gitlab-test

If using Docker, you should receive the error message introduced in this MR. Docker alternatives do not bubble up the error, this is a known/accepted problem.

Alternatively you could test this using curl e.g.:

curl -u username:password --request GET \
  --url 'http://gdk.test:3000/jwt/auth?client_id=docker&service=container_registry&scope=repository%3Agitlab-org%2Fgitlab-test%3Apush' 

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.