Update the docker tag revision to force rebuild

Author: embray

Dockerfile

@@ -1,4 +1,4 @@
-FROM sagemath/sagemath:8.0-1
+FROM sagemath/sagemath:8.0-2
 # Inspired from https://mybinder.readthedocs.io/en/latest/dockerfile.html#preparing-your-dockerfile
 # Make sure the contents of our repo are in ${HOME}
 COPY . ${HOME}