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/

• Add VOLUMEs to Dockerfile
• Removed all other Dockerfiles (moved everything from docker/single/ to docker/)
• Create DockerHub repository for GitLab CE
• Reword README to use single image
• 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.