Silly me, I missed an already existing bug report...
https://github.com/sailfish-containe...iners/issues/5 aka "Container cannot be created without a device lock"...

I added a setup lock, and after some tries (needed to reboot, to launch containers from icon and not from command line to try to get debug info...), it now allows to create a new container !

Will probably not finish today (already late there), but the UI seems to work so far, launching terminal for the setup scripts like configuring Xsession.

Looking good !