This MR brings in a GitLab-CI pipeline.

The pipeline consolidates a docker builder, then sequentially builds kstars and runs all tests, including UI tests.
Because the size of the build folder is large, the pipeline uses a single stage to run both build and test.
docker/Dockerfile provides a builder image, currently stored as tallfurryman/kstars-ci.

How to record the activity of the test through xvfb-run:

xvfb-run --listen-tcp --server-num 42 -s "-ac -screen 0 1920x1080x24" \
    bash -c 'sleep 1 ; QT_QPA_PLATFORM=eglfs make test ARGS="-VV" ; sleep 15' & \
ffmpeg -f x11grab -video_size 1920x1080 -i :42 -codec:v libx264 -qp 0 -preset ultrafast ~/test_record.mp4

This is a single-line command producing two bash jobs. The tests run in the background, and the recorder on foreground.
Press 'q' to terminate ffmpeg properly during the 15 seconds given at the end of the test. Video is recorded to ~/test_record.mp4.v