Remove Python 3.6 testing, and use the CERN proxy for docker images
Remove Python 3.6 testing, and use the CERN proxy to remove the possibility of useage quotas from dockerhub.
GitLab jobs artifacts no longer stored forever as of Monday, November 4th 08:30h. Further information under https://cern.ch/otg0152576
Remove Python 3.6 testing, and use the CERN proxy to remove the possibility of useage quotas from dockerhub.