Mount volume with code from repository in services

We need to mount volume that contains the code that we want to test/we use for tests inside services. We need that for https://gitlab.com/gitlab-org/gitlab-qa/merge_requests/1.

gitlab-ce-test:
  variables:
    HOSTNAME: gitlab-qa.test
    GITLAB_URL: http://$HOSTNAME
  image: docker:1.11
  services:
   - docker:1.11-dind
  script:
    - docker network create --internal test
    - docker run -d --name gitlab-qa --net test --hostname $HOSTNAME gitlab/gitlab-ce:latest
    - docker run -t --net test -e GITLAB_URL:$GITLAB_URL -v $(pwd):/tests ruby:2.2 /tests/bin/run

In case like above /tests directory is empty, because when we execute docker commands this gets populated to dind service but there are no files available.

As discussed with @tmaczukin we can fix this problem soon.

/cc @stanhu