WIP: executor_docker: support gitlab registry tokens.

This allows the runner to retrieve images from the Gitlab registry using the CI token it has received from Gitlab.

Previously, the CI token was only useful for pushing images, but there are some use cases that are not covered by this, see #1473 for some of them.

The design of this particular solution isn't great because it is effectively a "hail Mary" after all other authentication options have failed. Ideally, Gitlab would inform us of the URL for the integrated registry (if any), so we could test for it when retrieving authentication information from configuration files, etc. Therefore, I'm leaving this as a WIP until I can get some feedback on the design, before I start figuring out how to write tests for it.