WIP: Run unittests on fixed Python image in pipeline

.gitlab-ci.yml

@@ -54,10 +54,11 @@ pylint:
   allow_failure: true
 
 # run unit tests
-unittest:
+unittest_py3.8:
   tags: [ docker ]
   stage: test
   needs: [ ]
+  image: python:3.8
   script:
     - touch ~/.pycaosdb.ini
     - make unittest