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