Runner register: Don't add `/cache` to runners.docker.volumes, when disable_cache=true

When registering a runner, GitLab automatically adds /cache to the volumes section in the resulting config.toml, even if disable_cache is true, for no reason

This has no use, but is hard-coded when registering a runner and causes this warning message:

WARNING: Container based cache volumes creation is disabled. Will not create volume for "/cache"