Support shared builds volume (docker executor)

Reasons for shared /builds volume are outlined in https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/66#note_3222227.

This MR does not introduce any configuration options, instead it checks whether /builds is mounted as a host volume and if it is - sets shared_builds_dir to true & prevents /builds/{group} volume from being created.

Usage:

docker exec -it gitlab-runner gitlab-runner register --non-interactive --description <description> --url http://gitlab.example.com/ci --registration-token <registration-token> --executor docker --tag-list docker --docker-image <docker-image> --docker-volumes /var/run/docker.sock:/var/run/docker.sock --docker-volumes /builds:/builds

Let me know if you want anything to be changed. Thank you.