title: What does working_dir tag mean in a docker-compose yml file
body: <p>Just wondering what the working_dir file really does for an image being loaded via docker-compose. A sample docker-compose.yml file as follows:</p>
  
  <pre><code>dev:
      extends:
          file: common.yml
          service: workspace
      volumes:
          - $ATOMSPACE_SOURCE_DIR:/atomspace
          - $COGUTILS_SOURCE_DIR:/cogutils
          # Uncomment the following lines if you want to work on moses
          # - $MOSES_SOURCE_DIR:/moses
      working_dir: /opencog # This is the same as the volume mount point below
      links:
          - postgres:db
          - relex:relex
  
  postgres:
      image: opencog/postgres
      # Uncomment the following lines if you want to work on a production
      # system.
      # NOTE: The environment variable `PROD` is set `True` then the entrypoint
      # script in opencog/postgres does additional configurations.
      # environment:
      #     - PROD=True
  
  relex:
      image: opencog/relex
      command: /bin/sh -c "./opencog-server.sh"
  </code></pre>