Don't require `docker login` to GitLab Container Registry

Description

We currently require CI scripts to do a docker login to the container registry, passing the pre-defined variable for the password. We should remove the need for this step.

Proposal

Options:

1. Auto-login all jobs
2. Some other login/authentication mechanism?
3. Create a plugin for docker actions that does the login

Notes

This issue is primarily about docker usage within a script, but gitlab-ce#20716 brings up authentication needs for image itself. (1) might solve both problems depending on how it's implemented, but likely not since the docker daemon of the runner is not (usually) the same daemon as the build job.

Links

• Related: gitlab-ce#20716