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
Status Name Job ID Coverage
  Test
passed test_py_tools_ds #39404

00:00:13

63.0%
passed test_styles #39397

00:00:07

failed test_py_tools_ds #39396

00:00:06

58.0%