Simple script to run/cron on your Nextcloud instance to either check and notify by email if an update is available or even perform the upgrade silently. This is never without risk of course and you should maintain proper backups before running an upgrade. An improperly finished upgrade can brick your Nextcloud instance for exemple simply when running out of diskspace.
It might have been better to write - at least parts - in PHP for instance to retrive the current version from the config/config.php file but it works as is in bash but might break if updater.phar at one point alters the way return messages are displayed.
Make sure user rights are correctly applied (i.e. 664 on files and 775 on dirs) and files owned by your webserver or the upgrade process will fail (this script does not specifically catches such errors if you let it run the update rahter than notifying only).
This supposes your system should be able to handle the mail command with appropriate configuration. I.e. with exim4 or postfix. But IMHO every one of your servers/services should be able to handle that in order to send other alerts and notification...