title: How to speed up Docker build
body: <p>I'm trying to create a Dockerfile for a project I'm working on. Installing all the required packages through <code>apt</code> and <code>pip</code> takes a couple of minutes. Since the required packages don't change, is there a way so I can skip to the steps that do change? </p>
  
  <p>I'm running Docker CE on OS X (version 17.06.2-ce-mac27).</p>