title: docker compose variable substitution
body: <p>It seems that I am not understanding something about variable substitution in the following page (my variable <code>NUM</code> is not registering): <a href="https://github.com/compose-spec/compose-spec/blob/master/spec.md#Interpolation" rel="nofollow noreferrer">https://github.com/compose-spec/compose-spec/blob/master/spec.md#Interpolation</a></p>
  <p>See screenshot below. Running this on mac OSX.</p>
  <p><a href="https://i.stack.imgur.com/WDS8J.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WDS8J.png" alt="docker-compose up" /></a></p>