dockerfile: copy `install.sh` to `/var/cache/gitlab-ci-multi-runner/`
dockerfile: copy `install.sh` to `/var/cache/gitlab-ci-multi-runner/`

outcoldman/docker-gitlab-ci-multi-runner,sameersbn/docker-gitlab-ci-multi-runner,oomathias/docker-gitlab-ci-multi-runner,munnerz/docker-gitlab-ci-multi-runner