Snap packages can easily be run in unpriviledged LXC containers. The steps to take are to configure snap in the container and install the snap package.
Create/start the container:
lxc launch ubuntu: nextcloud lxc exec nextcloud bash
Install snap in the container:
apt update && apt upgrade -y # update apt install -y squashfuse snap # install the snap utilities
Install the snap package:
snap install nextcloud