docker docker0 and container broadcast addresses not set
if you are passing NET_ADMIN to the Docker container, I would not use the docker0 network at all for your application.
If I understood correctly what you are trying to do, the UDP broadcast heartbeating on a known port is used by Docker containers that belong to different hosts to find each other, and not by different docker containers in the same host.
I would then recommend to use --net=host:
docker run --net=host --cap-add NET_ADMIN ....
Like this if you get a shell into the docker container, you will see that the network environment is exactly the same one of the host that is running the containers. If your application was running on that server earlier using UDP broadcast, it will work exactly in the same way in the docker container.