Oh wow, hit some limit and had to wait 20 hours before I could answer. Is that the Discourse default or specific to this installation?
Anyway, back to topic:
Just to be clear, I’m not talking about a pre-bootstrapped image. I forgot about the Kitematic use case for a moment, sorry about that. But yeah, for that use case a try-discourse image, which is the base image + precompile, is the way to go, I think. Just have the Dockerfile have an extra RUN that is the precompile, I suppose. With a git(hub) hook, the try-discourse image can be created automatically whenever a new Discourse version is released. But that’s not news to you, so I should stop 
. But having a more self-contained Discourse Docker image will simplify the try-discourse / Kitematic problem significantly.