[Docker] add matplotlib

Dockerfile

@@ -10,6 +10,7 @@ RUN rm requirements.txt
 RUN apt update; apt install nano
 RUN pip install jupyter
 RUN pip install netron
+RUN pip install matplotlib
 
 # Note that we expect the cloned finn directory on the host to be
 # mounted on /workspace/finn -- see run-docker.sh for an example