Plugins and custom images still need to be built, but for vanilla building for compose is becoming within reach these days - I’ve made a little proof of concept experiment here for anyone curious.
Obviously, since these are all my repos (not Discourse’s™) this is not anything Official, so YMMV, and definitely do not expect official support 
I’m casting this here, since it’s been mentioned folks are already running bitnami and other non-sanctioned ways of running.
The off the shelf postgres (or pgvector) images do not work for Discourse - Discourse needs to change the default config, does not connect via a superuser, support locales, and needs to support postgres upgrades without a Discourse admin needing to dump/restore.