I can't seem to reproduce the issue you are seeing. But I think I might know of a way to work around it.
Log in as the user you are trying to use for the Unity8 LXC session and open a terminal in a normal session or do this from a VT :
$ touch ~/.config/ubuntu-system-settings/wizard-has-run
This *should* keep the unnecessary wizard from running.
I can't seem to reproduce the issue you are seeing. But I think I might know of a way to work around it.
Log in as the user you are trying to use for the Unity8 LXC session and open a terminal in a normal session or do this from a VT :
$ touch ~/.config/ ubuntu- system- settings/ wizard- has-run
This *should* keep the unnecessary wizard from running.