Nextcloud Update Command Line
Instead of clicking the button in the web interface you can now invoke the command line based updater by going into the /var/www/nextcloud/updater/ directory and executing the updater.phar as the web server user.
cd /var/www/nextcloud/updater/;sudo -u www-data php updater.phar
You will want to do the occ upgrade from the command line anyway to avoid timeouts through the web interface so you might as well do the whole thing from command line.