Is anybody able spotting, what am I doing wrong here? Thank you
Unless I'm mistaken, it seems like you are trying to run jetty and postgres in the same container. You should really use two containers and run them both with docker compose. Docker is very much designed to run a single service. This should also greatly simplify your Dockerfile - you could probably just use the vanilla postgres image for the postgres container. Here's some info on how to achieve this:
