Florian noticed that there were some problems with the docker build on master. As usual, they were related to python incompatibilities - so annoying. I've pushed a fix to 3.3-dev onward so that all three release branches continue to use the same docker image which is convenient I suppose. I think this could have been fixed more easily by just updating the master branch to bionic from xenial but eh...i dunno, decided to try to keep the same image across the board instead.
Please regenerate your docker images to get the fix.
