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.

• https://gitlab.com/gitlab-org/gitlab-ce/issues/19219#note_35675482

/cc @bikebilly @axil

mentioned in issue #19219 (closed)

changed the description

mentioned in issue #34356 (moved)