Update Dockerfile

Dockerfile

@@ -3,11 +3,12 @@ FROM ubuntu:18.04
 # Avoid user interaction dialog
 ENV DEBIAN_FRONTEND=noninteractive
 
-RUN apt -y update 
-RUN apt install -y git python3 python3-pip gcc psutils cmake wget make
+RUN apt -y update
+# Binder fails with Cython from pip -> use apt cython3
+RUN apt install -y git python3 python3-pip gcc psutils cmake wget make cython3
 RUN apt install -y gfortran libblas-dev liblapack-dev python3-tk sudo fonts-lato
 RUN pip3 install pip -U
-RUN pip3 install cython matplotlib pytest scipy psutil pyvtk ipywidgets -U
+RUN pip3 install matplotlib pytest scipy psutil pyvtk ipywidgets -U
 RUN pip3 install --no-cache-dir notebook
 
 RUN ln -s /usr/bin/python3 /usr/bin/python