Add docker images to .gitlab-ci.yml, Contributed by Simon Hanna (!72)

This commit tries to streamline the process of running the continuous integration. This is a slight modification of !72 (closed) by Simon Hanna to use a different docker image. The docker image is there on github and can run tests for all the different databases. The reason to host it on github was that it dockerhub allows automatic builds from github repos.

This would allow all contributors to use the Gitlab General runners to build their forks without setting up a runner of their own.