The single-worker serve path honored --host, but run_multi_worker_serve hardcoded the load balancer to 127.0.0.1. With --host 0.0.0.0 and --workers >1 (the common Docker setup) the balancer bound loopback only, so the mapped port was refused from outside the container. Bind the balancer to the requested host. Workers stay on loopback and are reached only by the balancer.
Closes#336.