If you get this error

Jul 14 13:00:20 mariadb systemd[1]: Starting MariaDB 10.3.15 database server...
Jul 14 13:00:21 mariadb systemd[1296]: mariadb.service: Failed to set up mount namespacing: Permission denied
Jul 14 13:00:21 mariadb systemd[1296]: mariadb.service: Failed at step NAMESPACE spawning /usr/sbin/mysqld: Permission denied
Jul 14 13:00:21 mariadb systemd[1]: mariadb.service: Main process exited, code=exited, status=226/NAMESPACE
Jul 14 13:00:21 mariadb systemd[1]: mariadb.service: Failed with result 'exit-code'.
Jul 14 13:00:21 mariadb systemd[1]: Failed to start MariaDB 10.3.15 database server.

The workaround is to override the mariadb.service file

cat <<EOF >/etc/systemd/system/mariadb.service.d/override.conf
[Service]
ProtectSystem=off
PrivateDevices=false
ProtectHome=false
EOF

If running a container in Proxmox you can also enable Nesting from web gui container > Options > Edit > Features > Nesting