Hi nihe, thank’s for the input 
Since I’m using Docker, I’m not sure were/how to issue that command. On the server running Docker? Probably not.
In the Nextcloud container? Does not work:
root@605df339b705:/var/www/html# sudo -u www-data php occ upgrade -v
bash: sudo: command not found
I guess this is probably really simple, but right now I’m clueless…