Update docker documentation to use only one docker container

I updated documentation to use only one docker container with manually specified path to the persistent data. The container is named `gitlab` and the image will be put in GitLab organisation: https://registry.hub.docker.com/u/gitlab/gitlab-ce/

- [x] Add VOLUMEs to Dockerfile
- [x] Removed all other Dockerfiles (moved everything from docker/single/ to docker/)
- [x] Create DockerHub repository for GitLab CE
- [x] Reword README to use single image
- [x] Push images (manually) to the repository
- [ ] Push to GitHub
- [ ] Enable Automated Build
- [ ] Integrate tagging and pushing of the image with GitLab CI

@JobV @sytses Please review.

See merge request !909