Add a cron job for Nexctloud in a docker

Hi there !
I installed Nextcloud in a docker and it's working perfectly fine : )
But now, i'ld like to make Preview Generator work, wich needs to add a cron job. I understand (do I ?) how a cron job works, but I don't understand how to set it up.
I have read this, but didn't understand...
Can someone explain me step by step ?
Thanks

I tried, but when I run this command :
docker exec --user www-data nextcloud php cron.php

I got this :
unable to find user www-data: no matching entries in passwd file

and when I tried this :
docker exec --user root nextcloud php cron.php
or
docker exec --user abc nextcloud php cron.php

I got this :
Could not open input file: cron.php

"-user abc", because it's said so in Nextcloud :

Use system cron service to call the cron.php file every 15 minutes. The cron.php needs to be executed by the system user "abc".

[EDIT] :
I managed to find the right path to "cron.php", but when I use the command "crontab -e", it says "no crontab for root - using an empty on ".
Not sure it's normal. Can someone confirm please ?