Used this https://spectechular.de/post/nextcloud_mit_docker_installieren/ but needed to change a few lines of that, cause there isn’t a current default maria image(?) for raspberry.
So I just changed that line to:
image: jsurf/rpi-mariadb:latest
That worked for me. More information on that image at: https://hub.docker.com/r/jsurf/rpi-mariadb/
Pay attention to just enter “nextcloud-db” as database host in the nextcloud installation routine.