feat: configure dnscluster to allow multi-node

* Configure `docker-compose` to scale at least 2 nodes.
* Add script to start service with proper naming
* Create custom resolver.
  * This is not strictly necessary.

priv/docker/service/docker-entrypoint.sh

@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+elixir --name ${HOSTNAME}@$(hostname -i | cut -d ' ' -f1) --cookie ${1} -S mix phx.server