Cleanup dockerfile a little

- Don't do pip install -U pip - this makes the image build more
  non-deterministic than necessary. 17.04 has the latest version of
  pip, and we can stick to newest ubuntu quite aggressivley to keep
  up in the cases it matters.
- Set --no-cache-dir in pip install, so we don't actually build
  wheels - they are useless in our situation. This also lets us not
  have to install wheels
- Make note about

Dockerfile

@@ -9,12 +9,11 @@ RUN apt-get update && \
             git && \
     apt-get clean && apt-get purge
 
+# FIXME: Remove this if pip install with setup.py can do it instead?
 COPY install-s2i.bash /usr/local/bin/install-s2i.bash
 RUN /usr/local/bin/install-s2i.bash
 
-RUN pip3 install --upgrade pip wheel
-
 RUN mkdir /tmp/src
 ADD . /tmp/src
-RUN pip3 install /tmp/src
+RUN pip3 install --no-cache-dir /tmp/src