Follow-up from "Document dependency proxy and container registry auth for cells"

The following discussion from !10394 (merged) should be addressed:

• @hswimelar started a discussion: (+2 comments)

  Unless something has changed, I believe you can auth with your user account credentials as well

Currently, our documentation for the container registry includes four methods to authenticate. A user's GitLab username and password are not on the supported list, although they are on the list for the dependency proxy, which uses the same authentication endpoint.

While we probably should not recommend using username + password to authenticate with the registry, since tokens are more secure, trackable, and rotatable, we may want to document whether the combo can be used to authenticate with the registry.