Hey. Good questions. I don’t think it would be a good idea to minimize the repo though, because you’re still asking for a lot of things that you need but others won’t 
(notably: dreambooth, s3, and runtime downloads, are 3 very big features that most people will never use).
If you wanted to go that route, which I totally get, I’d say rather than minimizing docker-diffusers-api, rather maximize banana’s starter repo. Because then it will be much clearer what’s going on and you’ll only be adding what you directly need. But it’s a lot of work.
Don’t forget that even banana’s super simple starter template is built around diffusers library, which is huuuuuge and covers everything (docker-diffusers-api is just a fancy wrapper around it).
The truth is though, you really don’t need to know / understand all parts of docker-diffusers-api or even diffusers, for the simple use cases. e.g. all the build-arg defaults are enough to get started pretty quick, although I admit it would help to have a default scheduler and pipeline, and that’s something we could add quite easily.
However, these really are things you’ll probably want in the future… the schedulers can have quite a big impact on the quality of certain types of images and models (and of course the latest scheduler, gets great results in just 20 (!) steps instead of 50, so you’re more than halving your image generation time and GPU cost). Pipelines, especially the community pipelines, add amazing features like weighted prompts (if you see everyone using e.g. “girl with ((big eyes))” everywhere, to emphasize the big eyes). But yeah, you shouldn’t need to understand this stuff to get started.
Hope that helps, not sure how clear my answer was, it’s nap time now haha. But P.S., runtime download and management of models (instead of during the build step) is being actively worked on now (it already works for dreambooth models).
P.S. What’s with the Japanese reference, any personal interest? (I’ve been learning Japanese for years (well, slowly on Duolingo at least) and I’m trying to structure my year to spend more time there).