Thanks!!!
Edit:
More answers about cron for nextcloud docker:
nevertheless after i wrote the comment above i found this article in german https://www.projekt-rootserver.de/cron-events-in-docker-containern-zum-laufen-bringen/2019/09/ it says: don’t do it with busybox. not a good idea. the authors advice: use the host cron + docker exec
Source: Docker setup & cron - #8 by Reiner_Nippes