docker-compose: define `GITLAB_CI_PORT` environment variable
docker-compose: define `GITLAB_CI_PORT` environment variable

bboehmke/docker-gitlab-ci,youhong316/docker-gitlab-ci,sameersbn/docker-gitlab-ci,youhong316/docker-gitlab-ci,sameersbn/docker-gitlab-ci,outcoldman/docker-gitlab-ci,bboehmke/docker-gitlab-ci,outcoldman/docker-gitlab-ci,adiferd/docker-gitlab-ci,adiferd/docker-gitlab-ci