Invalid generated `$CI_REGISTRY_IMAGE`

As you can see, my username has a trailing hyphen. Thus, my container registry is apparently located at registry.gitlab.com/magneticflux-. However, the Docker tag spec states that:

Name components may contain lowercase letters, digits and separators. A separator is defined as a period, one or two underscores, or one or more dashes. A name component may not start or end with a separator.

This causes me to be unable to push containers to my registry:

$ docker build -t "$CI_REGISTRY_IMAGE:base" -f Dockerfile.base .
invalid argument "registry.gitlab.com/magneticflux-/docker:base" for "-t, --tag" flag: invalid reference format
See 'docker build --help'.

This is related to #271583 (closed) and #343201, but regarding a different form of normalization.

added groupauthentication and authorization [DEPRECATED] label

added devopsgovern sectionsec labels

@adil.farrukh I assigned this to ~"group::authentication and authorization" because of the similarlity to #423030 (closed)

another case where folks aren't free to pick any username that's permitted.

@bprescott_ Thanks! I am on the fence about the correct path here. On one hand the docker spec dictates the supported paths for container registry names which can't really be changed easily, but on other hand I'd also not want to stop users from including a trailing hyphen in the username because from a sign-in/login perspective there isn't a harm in doing so.

Perhaps we add additional messaging both in product and docs to point users to a few examples where using a trailing hyphen can be problematic at the time of username creation? And let them make the decision after that?

added groupauthentication label and removed groupauthentication and authorization [DEPRECATED] label