Hi Julian, great work!!

I've also been working (interruptedly) for a couple of months on a docker version of Koha. 

I focused on having as soon as possible a fully functional docker version of Koha, and didn't worry too much about doing things the docker way.. It's comforting to know you weren't tempted by de dark side of the force!

I thought of having first a production ready docker much like koha-testing-docker, and later split services into different containers. That's why I really, really loved your implementation of plack.psgi. URL rewrite in psgi brings independence day from Apache much closer! \o/.

Just some thoughts:

• Should you base your image on Debian buster? on https://wiki.koha-community.org/wiki/Koha_on_Debian says "Note: Debian 10 (Buster) is currently not recommended or supported, work is in progress to rectify this situation."
  
• Even if there's no need for reverse proxy, I wouldn't discard having one on a separate container.. web servers like apache or nginx deal much better with attacks than exposing the service on it's own (First hand experience with a Java/Tomcat service I made once) (https://security.stackexchange.com/questions/209111/security-benefits-of-reverse-proxying-java-tomcat-webapps-locally)

But overall, great job!

PS: If you'd like to take a peek of what I've done, here's the repo: https://gitlab.com/thekesolutions/koha-docker