sorry, the original instructions detailed in the first post of the thread I hijacked....
Yes, I realize the job of the dhcp relay is to 'relay' the dhcp request it receives on port 67 to some backend dhcp server...presumably on port 67 too or maybe another port <--this part I cannot seem to figure out.
You will see in those original instructions the pihole container is indeed ran in bridge mode and port 67 is not mapped. The separate container (dhcpehelper) is running the relay and is listening to port 67 on host using 'host' networking and is forwarding these requests to the pihole container on the docker bridge. There is something that I am missing that is causing I think the dhcp requests to be received on the pihole container and/or clients do not receive the reply.