set up Gitlab pipeline for building a data xar package and a Docker image

This entails

1. creating an ANT build.xml along with necessary EXPath files for building a EXPath xar package (which can be deployed in an eXist database)
2. creating a Dockerfile for building a Docker image
3. creating a .gitlab.yml file for automated processing of the pipeline

I assume that the provided shared runner "shared_runner_demo_04" is able to execute Docker builds and that we may store artifacts as well as Docker images in the Gitlab registry?!