Simplify docker files

docker/Dockerfile-pythran

 FROM python:3
 MAINTAINER Matthieu Boileau <matthieu.boileau@math.unistra.fr>
 
-COPY requirements.txt ./
-RUN pip install --no-cache-dir -r requirements.txt
-
 RUN apt-get update --fix-missing && \
       apt-get install -y \
+      python-dev \
       vim
 
 RUN apt-get clean && \
 rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
+RUN pip install --no-cache-dir virtualenv
+RUN pip install --no-cache-dir --upgrade pip
+COPY requirements.txt ./
+RUN pip install --no-cache-dir -r requirements.txt
+
 RUN useradd -m -s /bin/bash  euler
 ENV HOME /home/euler
 RUN chown -R euler:euler /home/euler
 USER euler
 WORKDIR $HOME
 
-CMD /bin/bash
+RUN virtualenv --python=/usr/bin/python2.7 $HOME/py27
+COPY requirements.txt ./
+RUN $HOME/py27/bin/pip install --no-cache-dir -r requirements.txt
 
+CMD /bin/bash

docker/docker-compose.yml

 version: '2'
 
 services:
-  virtualenv:
-    build:
-      context: ../
-      dockerfile: docker/Dockerfile-pythran-virtualenv
-    image: boileaum/pythran:virtualenv
 
   env:
     build: