Prepare to let jenkins master-builds use debian-bullseye-jenkins instead of debian-buster-jenkins. Add almost everything from the buster Dockerfile to the bullseye Dockerfile that is not there, except for: dh-systemd (not available in debian11)
Also add python2, as some projects still require it to run tests (OS#5950).
Add a bullseye based container that can be used instead of debian-buster-jenkins-arm. The master-builds jenkins jobs will attempt to use this jenkins job for the arm builds after setting the main default image from debian-buster-jenkins to debian-bullseye-jenkins.
