title: context or workdir for docker-compose
body: <p>I'm learning docker</p>
  
  <p>I need to specify the working directory for a docker image, I think that'll be something like this:</p>
  
  <pre><code>version: '2'
  services:
    test:
      build:
        context: ./dir
  </code></pre>
  
  <p>Now I want to make the image <code>python:onbuild</code> to run on the <code>./dir</code>, but I dont want to create any <code>Dockerfile</code> inside the <code>./dir</code>.</p>
  
  <p>The <code>docker-compose</code> manual says nothing about that.</p>
  
  <p>Is it possible? How to do that?</p>