Reboot ALP host and 'podman run' gdm container erases root user's application

[cachen]

Steps to reproduce:
By following https://confluence.suse.com/display/~cachen/Experience+of+ALP+with+GDM+container
1, successful installed alp host and gdm container
2, root user, successful load gdm and gdm container by running "podman container runlabel --name gdm run registry.opensuse.org/suse/alp/workloads/tumbleweed_containerfiles/suse/alp/workloads/gdm:latest"
3, login as root
4, successful install Firefox from flathub by running "flatpak --user "
5, reboot alp host, root user load gdm container by repeat step 2
6, Firefox is erased

Discussed with Yifan, the issue caused by podman run will create a new gdm container which is expected, and podman start the old one could keep firefox alive. However the root user will have the same problem when we have image upgraded, how to store root user's application is a topic for discussing

[yifanjiang]

The /root is not bound to host, so we do not have a way to manage the pacakge installed by "flatpak --user" being permanent in this case. Meanwhile it's worthy of discussing if we support root user login in the gdm-container and keep its installed applications.