Update Dockerfile-pythran

docker/Dockerfile-pythran

@@ -9,10 +9,10 @@ RUN apt-get update --fix-missing && \
 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 pip install --no-cache-dir virtualenv
+COPY requirements.txt /tmp/
+RUN pip install --no-cache-dir -r /tmp/requirements.txt
 
 RUN useradd -m -s /bin/bash  euler
 ENV HOME /home/euler