title: Unable to upgrade pip in docker build
body: <p>In running the Docker build (using Jenkins CI), it fails on upgrading pip (last line of the docker file).  I need it to upgrade version 8.1.1, as it suggest in the log, as my deploy fails on PIP versions mismatch.</p>
  
  <p>Dockerfile </p>
  
  <pre><code>FROM ubuntu:14.04
  FROM python:3.4   
  
  # Expose a port for gunicorn to listen on
  EXPOSE 8002
  
  # Make a workdir and virtualenv
  WORKDIR /opt/documents_api
  
  # Install everything else
  ADD . /opt/documents_api
  
  # Set some environment varialbes for PIP installation and db management
  ENV CQLENG_ALLOW_SCHEMA_MANAGEMENT="True"
  
  RUN apt-get update
  RUN apt-get install -y python3-pip
  
  RUN pip3 install --upgrade pip
  </code></pre>
  
  <p>Here's the error:</p>
  
  <pre><code>Step 15 : RUN pip3 install --upgrade pip
  19:46:00  ---&gt; Running in 84e2bcc850c0
  19:46:04 Collecting pip
  19:46:04   Downloading pip-8.1.1-py2.py3-none-any.whl (1.2MB)
  19:46:04 Installing collected packages: pip
  19:46:04   Found existing installation: pip 7.1.2
  19:46:04     Uninstalling pip-7.1.2:
  19:46:05       Successfully uninstalled pip-7.1.2
  19:46:10 Exception:
  19:46:10 Traceback (most recent call last):
  19:46:10   File "/usr/local/lib/python3.4/shutil.py", line 424, in _rmtree_safe_fd
  19:46:10     os.unlink(name, dir_fd=topfd)
  19:46:10 FileNotFoundError: [Errno 2] No such file or directory: 'pip'
  19:46:10 You are using pip version 7.1.2, however version 8.1.1 is available.
  </code></pre>