title: First line of Dockerfile - what OS?
body: <p>Occasionally, I see the first line of Dockerfile as:</p>
  
  <pre><code>FROM ubuntu:16.04
  </code></pre>
  
  <p>And sometime as </p>
  
  <pre><code>FROM python:2.7-slim
  </code></pre>
  
  <p>Assume that I am running Docker on Ubuntu 18.04 and first line of my Dockerfile is  <code>FROM python:2.7-slim</code> does it mean that by default all other necessary resource will come from Ubuntu 18.04?</p>
  
  <p>Also, if I am running Docker on Ubuntu 18.04 and first line of my Dockerfile is <code>FROM ubuntu:16.04</code>. Does it mean that now all my necessary resources will come from Ubuntu 16.04?</p>