Hi guys, possible of my personal server configuration or what else.. But I found a solution by running a docker in hosted mode (not bridged as default):
/var/discourse/launcher rebuild app --docker-args --net=host --skip-mac-address
After that the discourse engine could connect to external postgres database, run at 127.0.0.1:5432. Default bridged mode does not allow docker to access ports listening at the host without tricks.
Hope this might help somebody.