Because they only support one installation type, which is in a clean VPS where all the modifications and scripts run without any issue. Anything else is a custom installation and they won’t support it as the ways to use containers is infinite.
To be able to install Discourse, you need to run the bootstrapping done via the launcher utility. If you can add scripts to Portainer it should be relatively easy to do. Or you can just install Discourse in a VPS and push the resulting container to see if that works.
Other way, which could be easier is to decouple everything and deploy them individually, this means (in no way a complete list):
- PostgreSQL
- Redis
- Unicorns
- Discourse