title: Setting specific python version in docker file with specfic non-python base image
body: <p>I want to create a docker image with specifically python 3.5 on a specific base image which is the nvidia/cuda  (9.0-base image) the latter has no python environment.</p>
  
  <p>The reason I need specific versions is to support running cuda10.0 python3.5 and a gcc version&lt;7 to compile the driver all together on the same box </p>
  
  <p>When I try and build the docker environments (see below) I always end up with the system update files which load python3.6</p>
  
  <p>The first version I run (below) runs a system update dependencies which installs python 3.6  I have tried many variants to avoid this but always end up 3.6 in the final image. </p>
  
  <p>Any suggestions for getting this running with python3.5 are welcome</p>
  
  <p>Thanks</p>
  
  <pre><code>FROM nvidia/cuda
  
  RUN apt-get update &amp;&amp; apt-get install -y libsm6 libxext6 libxrender-dev python3.5 python3-pip 
  
  COPY . /app
  WORKDIR /app
  
  RUN pip3 install -r requirements.txt
  ENTRYPOINT [ "python3" ]
  CMD [ "app.py" ]
  </code></pre>
  
  <p>Another variant (below) I have tried is with virtualenv and here again I can't seem to force a python 3.5 environment</p>
  
  <pre><code>FROM nvidia/cuda
  
  RUN apt-get update &amp;&amp; apt-get install -y --no-install-recommends libsm6 libxext6 libxrender-dev python3.5 python3-pip python3-virtualenv
  
  ENV VIRTUAL_ENV=/opt/venv
  RUN python3 -m virtualenv --python=/usr/bin/python3 $VIRTUAL_ENV
  ENV PATH="$VIRTUAL_ENV/bin:$PATH"
  
  COPY . /app
  WORKDIR /app
  
  RUN pip3 install -r requirements.txt
  ENTRYPOINT [ "python3" ]
  CMD [ "app.py" ]
  </code></pre>