master is now 1.7.0: update the docker image to qt 6
1.7.0 needs qt 6 but there seems to be no (easy) way to get qt 6 on a debian based system.
we will need the qt 6 packages to hit debian testing and ubuntu (at least with a ppa) to provide again appimages.
until then you can get the latest appimage published by the scribus team.