Docker CI Development

The current Dockerfile does not contain instructions for a graphviz installation required by the pydot module. This is added with this MR. Additionally, layer complexity in Dockerfile is reduced by chaining RUN instructions.

Furthermore, a .gitlab-ci.yml script for automatic docker image building is added in this MR. Explanation for the runner configuration can be found here. The pipeline is currently set up to only execute if Dockerfile is modified in the master branch. In the future, additional triggers on new git tags could be added as well.

The docker images can be found afterwards in [https://gitlab.cern.ch/chscheul/mva-trainer/container_registry] after creation. They can then be downloaded directly by singularity for lxplus, for instance. This is especially convenient, as docker build is not available there.

ATTENTION: Some clean-up rules for the container registry should probably be added after this MR is merged.