Maybe this way?

.gitlab-ci.yml

 job-flake8-2:
-  image: mpcdf/osc-plugins/python2:flake8
+  image: python2:flake8
   script:
     - flake8 --filename=*.py --count --ignore=E501 .
 
 job-flake8-3:
-  image: mpcdf/osc-plugins/python3:flake8
+  image: python3:flake8
   script:
     - flake8 --filename=*.py --count --ignore=E501 .