I am trying to receive UDP messages via IPv6. My Node-RED is in a docker where I activated IPv6. I am able to receive UDP messages via IPv4 but not via IPv6.
I tried following command for port forwarding without success.
sudo ip6tables -A FORWARD -i eth0 -o docker0 -p udp --dport 1881 -m conntrack --ctstate NEW -j ACCEPT
THe tests you need to do are to get to a command line inside your docker then try to do udp to the host and visa versa. That way you are testing whether udp can cross the boundary at all (eliminating Node-RED as the possible problem).
I don't think it is your test or Node-RED that's wrong but rather your docker networking config. I'm afraid I'm no expert on docker though and you may need to ask in a docker support forum.