Document how to use persistent token during CI to `docker login`

From https://gitlab.com/gitlab-org/gitlab-ce/issues/19219#note_38448358:

  • it would be nice to have documentation on how to use this token. I'm trying right now and I don't see how I can make this work using docker login or docker pull.
  • Many people (including me) want to docker login using a deploy key but we don't know how to do it.
  • The solution is pretty simple in the end: docker login using the token as password. What you input as username doesn't matter. As I kinda stumbled into the solution, I'd rather have a dev do the doc than me.
  • Just put any values in the other fields. I set the username to docker and the email to docker@docker.com.
Edited by Jason Yavorsky