Just wanted to give you and my future self a nudge.
I resolved my issue using the unofficial Addon "Run On Startup.d", which i use to add a custom entry to the hosts file of the hassio_dns-container:
➜ ~ cat /config/startup/startup.d/hassio_dns.sh
#! /bin/bash
echo "This script is executed in the hassio_dns container";
env;
echo "192.168.30.190 SECRETHOSTNAME.duckdns.org" >> /config/hosts;
This way, everytime homeassistant boots, the Hostname gets an entry with it’s local IP.