Explicitely set ulimits for mq to ERL_MAX_PORTS (metabrainz#258)

On Linux ERL_MAX_PORTS defaults to 65,536. But the Docker container for
the service `mq` rather uses the host ulimits. When ulimits is infinite,
it can make the container to take a very long time to start while using
a lot of CPU for nothing. It depends on the host settings.

This patch explicitely sets ulimits to 65,536 as recommended for `mq`.

See docker-library/rabbitmq#545 for details.

docker-compose.yml

@@ -113,6 +113,8 @@ services:
         max-size: "10m"
         max-file: "10"
     restart: unless-stopped
+    ulimits:
+      nofile: 65536
     volumes:
       - mqdata:/var/lib/rabbitmq
     expose: