title: Host names are not set in docker compose
body: <p>I created simple compose config to try Postgres BDR replication.
  I expect containers to have host names as service names I defined and I expect one container to be able to resolve and reach another with this hostname. I expect it to be true because of that:
  <a href="https://docs.docker.com/compose/networking/" rel="nofollow">https://docs.docker.com/compose/networking/</a></p>
  
  <p>My config:</p>
  
  <pre><code>version: '2'
  
  services:
    bdr1:
      image: bdr
      volumes: 
        - /var/lib/postgresql/data1:/var/lib/postgresql/data
      ports:
        - "5001:5432"
    bdr2:
      image: bdr
      volumes: 
        - /var/lib/postgresql/data2:/var/lib/postgresql/data    
      ports:  
        - "5002:5432"
  </code></pre>
  
  <p>But in reality both containers get rubbish hostnames and are not reachable by container names:</p>
  
  <pre><code>Creating network "bdr_default" with the default driver
  Creating bdr_bdr1_1
  Creating bdr_bdr2_1
  Attaching to bdr_bdr1_1, bdr_bdr2_1
  bdr1_1 | Hostname: 938e0585fee2
  bdr2_1 | Hostname: 7153165f4d5b
  </code></pre>
  
  <p>Is it a bug, or I did something wrong?
  I use Ubuntu 14.04.4 LTS, Docker version 1.10.1, build 9e83765, docker-compose version 1.6.0, build d99cad6</p>