title: Can't install pip packages inside a docker container with Ubuntu
body: <p>I'm following the <a href="http://www.fig.sh/index.html">fig guide</a> to using docker with a python application, but when docker gets up to the command</p>
  
  <pre><code>RUN pip install -r requirements.txt
  </code></pre>
  
  <p>I get the following error message:</p>
  
  <pre><code>Step 3 : RUN pip install -r requirements.txt
   ---&gt; Running in fe0b84217ad1
  Collecting blinker==1.3 (from -r requirements.txt (line 1))
    Retrying (Retry(total=4, connect=None, read=None, redirect=None)) after connection broken by 'ProtocolError('Connection aborted.', gaierror(-2, 'Name or service not known'))': /simple/blinker/
  </code></pre>
  
  <p>This repeats several times and then I get another message:</p>
  
  <pre><code>Could not find any downloads that satisfy the requirement blinker==1.3 (from -r requirements.txt (line 1))
    No distributions at all found for blinker==1.3 (from -r requirements.txt (line 1))
  </code></pre>
  
  <p>So for some reason pip can't access any packages from inside a docker container. Is there anything I need to do to allow it internet access?</p>
  
  <p>However pip works fine to install things outside of the docker container, and worked fine even with that exact package (<code>blinker==1.3</code>) so that's not the problem. Also this problem isn't specific to that package. I get the same issue with any <code>pip install</code> command for any package.</p>
  
  <p>Does anyone have any idea what's going on here?</p>