Work around udocker bug in handling ENTRYPOINT

Dockerfile

 FROM centos:7
 MAINTAINER stefano.bagnasco@to.infn.it
+
 RUN yum install -y numpy python-matplotlib &&\
     yum clean all
 
 COPY mandelbrot.py /usr/lib/mandelbrot.py
-ENTRYPOINT ["/usr/bin/python","/usr/lib/mandelbrot.py"]
+RUN chmod a+x /usr/lib/mandelbrot.py
+
+ENTRYPOINT ["/usr/lib/mandelbrot.py"]
 CMD ["50", "50.", "601", "401"]
\ No newline at end of file