We made it work with, heavily inspired by Guillaume's reply. Some details about a major problem we faced and our solution:

To make this work with the IRIS Data Platform Docker Image, we had to edit the /durable/httpd/conf/httpd.conf of the Apache2 in the docker image to enable SSL (and generate the keys). If SSL terminates at a proxy that runs in a separate docker container (either nginx or apache) the OAuth2 endpoints did throw the following error: "Request must be secured with TLS/SSL". Thus SSL must terminate at the web server that runs the CSP Plugin/Module, in this case the apache2 in the IRIS Data Platform Docker image.