I was able to help the project get their dockerfile off the ground so that it builds and runs with just docker, but it still has a ways to go. Here are some things I think would need to be improved before it would be suitable for cloudron:
Reduce image size by copying binary results out of the build stages. This is a bit complex because the project is built with rust/wasm-pack and runs a wasm on the frontend, and uses haskell stack for the backend. I'm not familiar with doing docker optimizations with either of these technologies.
Change the backend to save all data into a /data volume. This will require some changes in the haskell code.
Get deps-only build steps working. This would be a nice-to-have for the devs to reduce build times after the first build (which can reach to 40min!), but this one isn't a deal breaker for cloudron. I tried, but it causes some compilation output issues so it was commented out for now.
The author was very nice and quite receptive to changes. If anyone else works on this you may want to just aim at getting them merged upstream.