Using workspaces to define a default layout including an open notebook worked fine. I just had to make sure that the config json was available to the docker build.
It still would be very useful to use urlpaths to point to a notebook since this does not require rebuilding the docker image.
Oh, that does work now, too: Binder
It looks like something got initialized by importing a workspace as a default workspace. What could that be ?
That leaves my other question about having to restart the kernel to make widgets render properly:
Perhaps the ipywidgets github issues is a better place for this question.