Docker Image mention

As per conversation with @dosire, I'l like to add the mention of the GitLab-CI Docker image I've build.
I'm not exactly sure how you'd like to word it or section to place it within, so feel free to throw this away and use your own wording...
Thanks GitLabHQ

Author: anapsix

README.md

@@ -79,6 +79,7 @@ and
 ### Installation
 
 * [Installation guide](doc/install/installation.md)
+* [Unofficial Docker Image](https://registry.hub.docker.com/u/anapsix/gitlab-ci/) is available via `docker pull anapsix/gitlab-ci` for testing and simplified installation.
 
 ### How to add a new project to GitLab CI