Ok I can confirm, the snap isolation was the issue for my case. (it might be the volume mount not the --cidfile as suspected, since that works and is not used)
Since Ubuntu is “still working on it” (or at least I could not find how to expose filesystems) and the --classic option also does not work for the docker snap:
snap remove docker && snap install --classic docker
Warning: flag --classic ignored for strictly confined snap docker
I went the “easy” route and used /home/discourse instead of /var/discourse as the base dir. (it does not even have to run as this user).
I can understand that you would require upstream docker, but on the other hand I can also understand Ubuntu to support their own version, so its a rock and hard place for users.