title: docker-compose ignores `configs:` section
body: <p>I've got a docker-compose service definition with a configs section as follows:</p>
  
  <pre><code># api.yml file
  version: '3.3'
  
  services:
    api:
      ...
      configs:
        - source: api_config
          target: /env.cfg
  
  configs:
    api_config:
    file: ./config/my.cfg
  </code></pre>
  
  <p>When I use this file to drive a swarm, i.e. I deploy it with <code>docker stack deploy ...</code> the config file is properly provided into the running containers.</p>
  
  <p>However, when I try to run a local version of the api via <code>docker-compose -f api.yml up</code> I get a startup failure complaining of my api service that it can't find its config. (NB: docker-compose itself doesn't report any errors.)</p>
  
  <p>Are <code>configs</code> sections not supported with plain <code>docker-compose</code> usage?</p>
  
  <p>The docs don't explicitly say so although some wording seems to indicate this is for stacks only:</p>
  
  <blockquote>
    <p>Note: The config must already exist or be defined in the top-level configs configuration of this stack file, or stack deployment will fail.</p>
  </blockquote>