title: Temporary failure in name resolution [Errno -3] with Docker
body: <p>I'm following the <a href="https://docs.docker.com/get-started/part2/" rel="noreferrer">docker tutorial</a> and am on the part where I have to build the app using:</p>
  
  <pre><code>docker build -t friendlyhello .
  </code></pre>
  
  <p>It reaches up to step 4, where after a pause I get this error:</p>
  
  <pre><code>Step 4/7 : RUN pip install -r requirements.txt
   ---&gt; Running in 7f4635a7510a
  Collecting Flask (from -r requirements.txt (line 1))
  
  Retrying (Retry(total=4, connect=None, read=None, redirect=None)) after 
  connection broken by
  'NewConnectionError('&lt;pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection 
  object at 0x7fe3984d9b10&gt;: Failed to establish a new connection: 
  [Errno -3] Temporary failure in name resolution',)': /simple/flask/
  </code></pre>
  
  <p>I'm not quite sure what this error means and how I can go about solving it.</p>
  
  <p>Thanks for your help! </p>