I would love to install it in the Foris interface, …, I already reported it as a bug.
We are aware of this issue and in the next release of Turris OS 4.0, it will be fixed.
For now, you can add following rows to /etc/config/updater:
config pkglists 'pkglists'
list lists 'lxc'
Just out of curiosity do you know why I do not see the possibility of creating LXC containers from REPO in LUCI interface but it work in SSH?
I will check it and let you know.