title: Download speed for pip install inside docker container very slow
body: <p>While creating a docker image from a dockerfile or being inside the created container, <code>pip install &lt;package&gt;</code> in a conda environment is super slow. </p>
  
  <p>The download speed hugely differs if I do the same without docker. 
  I tried to change the mirror <code>pip install -i http:pypi.doubanio.com/simple &lt;package&gt;</code> and can gain slight improvements, but still different inside and outside of docker.
  Testing the download speed inside the container via <code>curl -s https://raw.githubusercontent.com/sivel/speedtest-cli/master/speedtest.py | python -</code> gives me the expected good results.</p>