Mount /builds folder to all services when used with Docker Executor

This seems like a good default. I'm not sure about the mounting of other volumes, yet. This solves a specific use case:

image: docker:git
services:
- docker:dind

test:
  script:
  - docker run -v $(pwd):$(pwd) -w $(pwd) alpine bash run-script

This for now supersedes this MR: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/241