title: Composer install package from cache
body: <p>I'm using php composer 1.8.0 that I installed to docker <a href="https://github.com/docker-library/php/blob/1eb2c0ab518d874ab8c114c514e16aa09394de14/7.3/stretch/cli/Dockerfile" rel="nofollow noreferrer">php:7.3-cli</a></p>
  
  <p>Every time I install packages with this composer it downloads package, even the package was previously downloaded.</p>
  
  <ol>
  <li><p>Where does composer has its cache for the packages it downloaded?</p></li>
  <li><p>Is there any command line option or other to set a particular dir as a composer cache?</p></li>
  </ol>