Added some comments to build_testsuite_image.sh and set concurrency to 4....

Added some comments to build_testsuite_image.sh and set concurrency to 4. Updated deprecated code in .gitlab_ci.yml.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>
3 jobs for bugfix/fix_ci_connection in 21 seconds