Unable to build docker image

[jeffdoubleyou]

Deadsnakes PPA no longer supports Xenial so the build is now failing.

https://github.com/deadsnakes/issues/issues/195

I have managed to get it working by using:

-    add-apt-repository -y ppa:deadsnakes/ppa && \
+    add-apt-repository -y ppa:jblgf0/python && \