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Hi, I'm one of the devs.

That's totally true, but it means the python app can only run in the same container as the process. I thought there were essentially 2 really cool features of this: you don't have to clean up after your (sub)processes and your docker daemon could be running remotely. (e.g., you could distribute tasks to a bunch of servers running your containers)

Or, if you have docked installed on a system anyway why not just use docker-py (docker's python bindings) directly to run stuff in containers?

you can! I found it difficult to work with and decided to make something new after reading this post: http://blog.bordage.pro/avoid-docker-py/ It's a few years old but I think docker-py still needs some love for it to really shine.

I can see it being useful for building up a testing environment/framework

Absolutely, we use it especially because our servers are linux and our personal machines are mac. Most of our app is not containerized but sidomo helps us make the 'fiddly bits' (e.g., ffmpeg) super portable.