Allow user to specify custom docker labels for containers
Hi,
I would like to know if it's possible to add support for `--docker-label` in `gitlab-runner` register command.
I'm using rancher and would like to do something like this:
```sh
gitlab-ci-multi-runner register ... --docker-label io.rancher.container.network=true
```
So my runners would be able to resolve the rancher internal dns.
issue