hi @TurboC welcome to the community 
and like always - search helps - lot of issues have been discussed already!
this is not the right mount destination. From the docs you see
/var/www/html/folder where all Nextcloud data lives
you want to persist more than files only e.g. config is essential 
Nextcloud runs by default with user 33:33 I would recommend to assign this user and correct permissions (folders:750, files: 640) to all data mounted into NC container. Newer versions can run with a different UID but this requires more tweaking.