I’ve been fiddling with this a bit.
Using the Syno Docker install you can select the official Discourse/base
Map a share (/volume1/docker/discourse) to /var/discourse
make a symbolic link /var/discourse to the share
git pull works
./discourse-setup works
./launcher works (it complains about docker CE version, but stop/start/rebuild work)
Haven’t managed to get the web interface up, but it’s probably my own ignorance.