GitLab runners do not have `docker` tag

The GitLab runners that are being setup for our projects don't have a docker tag. They have docker-privileged but this doesn't match.

Oops - I see this is called dockerers now. This should be fixed here so that they are tagged with docker and docker-privileged (if they are also a docker privileged capable runner)

Can probably get rid of the code to tag them with arbitrary ansible groups/roles that have been applied. Now that we are using docker-machine, that won't matter anyway.

docker tag now present.