hi @HeinrichXVIII welcome to the forum 
looks you are running the cron job from your host and for this reason it can’t connect to the database. for testing purposes you should run it inside of your application container using docker command like
docker exec nextcloud php cron.phpordocker exec -u 33 nextcloud php cron.php
in long term you should setup a cron “sidecar” container
many topics exist in this forum related to the problem use search and check cron tag. see more advanced examples including cron: