[OMV6] Collabora Office Nextcloud

Hello, since yesterday I have been trying to install Collabora Online for Nextcloud which is hosted on my NAS under openmediavault 6.
For this I am looking for the simplest method.
I found this method using Portainer which I was already using with a docker-compose.

But I’m stuck at the part where he talks about CNAME because I don’t understand.
If anyone could help me please.