Update dockerfile to include conversion from python file to ipynb

Dockerfile

@@ -1,5 +1,5 @@
 # Use github pages for docker image
-FROM ghcr.io/scientificcomputing/example-paper-fenics:v0.3.0
+FROM ghcr.io/scientificcomputing/example-paper-fenics:v0.4.0
 
 # Create user with a home directory
 ARG NB_USER
@@ -11,6 +11,11 @@ ENV HOME /home/${NB_USER}
 WORKDIR ${HOME}
 COPY . ${HOME}
 
+# Convert demo.py into demo.ipynb
+RUN python3 -m pip install jupytext jupyter
+RUN python3 -m ipykernel install --name=python3 --user
+RUN python3 -m jupytext --to notebook --execute --output ./code/demo.ipynb ./code/demo.py
+
 # Change ownership of home directory
 USER root
 RUN chown -R ${NB_UID} ${HOME}