When my Gateway was installed as a deb package on my raspberry Pi 4, it recognised my own built webthing by domain name. This name is not in my pi4’s /etc/hosts file, so, as I do not run a DNS server on my LAN, it must have got the name via mDNS.
Now I moved the Gateway to a docker image using the host network (on a stock docker image) & some config/db.sqlite3 voodoo to connect to my existing account. I can access my own built webthings by IP, but not by domain name any longer.
So it seems that mDNS is broken in the docker image.