Add build pipeline

This MR adds a gitlab-based build pipeline. Build, test, and deployment for nightly builds operate correctly. After this MR has been merged, only the copy job for our file server has to be adjusted and the build trigger for the nightly build needs to set.

docker/Dockerfile

+FROM maven:latest
+
+RUN apt-get update
+
+RUN apt-get install -y libgtk-3-bin xvfb libgomp1
+
+ENV DISPLAY=:99
+ADD xvfb.sh /usr/bin/xvfb.sh
+RUN chmod a+x /usr/bin/xvfb.sh