add Dockerfile for demos

for easier reproduction without needing a full VM

demos/Dockerfile

@@ -0,0 +1,20 @@
+# Dockerfile for running / testing demos
+# build: `docker build -t nbgrader-demo .`
+# build and run: `docker build -t nbgrader-demo . && docker run --rm -it -p 8000:8000 nbgrader-demo`
+# run a demo without rebuilding by mounting the demo dir as a volume (e.g. if you are editing the demos):
+# docker run --rm -it -p 8000:8000 -v $PWD:/root nbgrader-demo /root/restart_demo.sh demo_multiple_classes
+
+FROM ubuntu:22.04
+ENV DEBIAN_FRONTEND=noninteractive
+RUN apt-get update \
+ && apt-get -y dist-upgrade \
+ && apt-get -y install git sudo
+# cache some of the bigger installs
+RUN apt-get -y install python3-pip nodejs npm
+RUN python3 -m pip install --upgrade jupyterhub notebook
+RUN git clone https://github.com/jupyter/nbgrader /srv/nbgrader/nbgrader
+# COPY is like deploy_demos.sh
+COPY ./ /root/
+WORKDIR /root
+CMD /root/restart_demo.sh demo_one_class_multiple_graders
+EXPOSE 8000