Document gitlab-runner ability to use private docker registry

The undocumented feature in question is gitlab-org/gitlab-ci-multi-runner!17.

Questions:

1. What the user has to add in config.toml, if anything? (Nothing)
2. Can you tell the runner what registry to use while registering it with GitLab CI? Is there a one-line? (This is configured by running docker login <registry>)
3. How are images called inside .gitlab-ci.yml? Does it change anything? (image: my.registry.tld:5000/namespace/image:tag)

This should go somewhere in advanced configuration and also mention the feature somewhere in the CI docs.

\cc @ayufan

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Notes to self:

• Mention that this was introduced with gitlab-runner 0.6.0
• Maybe link to https://docs.docker.com/registry/deploying/