title: For cabal, what does only-dependencies flag mean?
body: <p>I'm fairly new to Haskell, and upon seeing this flag, e.g. in this <a href="https://github.com/freebroccolo/docker-haskell/blob/master/examples/7.8/snap/Dockerfile" rel="noreferrer">dockerfile</a>, I can't ever seem to find an explanation for what it does. <em>"Install only the dependencies necessary to build the given packages,"</em> in the <code>cabal help install</code> doesn't say much to me.</p>
  
  <p>If I'm not building inside a docker container, I use sandboxes. Is this flag applicable to either of these situations?</p>