title: Docker-compose ssh agent forwarding
body: <p>I'm trying set up a ssh agent forwarding in a docker container mounting the path from the <code>SSH_AUTH_SOCK</code> env var in docker-compose.yml: </p>
  
  <pre><code>volumes:
    - "${SSH_AUTH_SOCK}:/keyring"
  </code></pre>
  
  <p>The docker-compose utility complains:</p>
  
  <blockquote>
    <p>Cannot create container for service app: create .: volume name is too short, names should be at least two alphanumeric characters</p>
  </blockquote>
  
  <p>Is <code>SSH_AUTH_SOCK</code> empty for docker-compose? How can I get this to work?</p>