Add runners.docker.container_labels

[Norskel]

Add support for defining docker container labels in GitLab Runner docker extra configs

https://docs.gitlab.com/runner/configuration/advanced-configuration/#the-runnersdocker-section -> container_labels

[guenhter]

Could you please have a look at the linting issues. Thx (maybe rebasing to the latests version also helps)

[guenhter]

LGTM. Thx

[joannakotula-enghouse]

gitlab_runner.extra_configs works good with other update modes. And, as I see, it works by iterating over extra_configs dictionary (see update-config-runner.yml line 1167.

I think that it should be similar in the by_template mode