Setup our CI configuration to run the Shadow integration tests in an Ubuntu 18.04 docker instance.

It is important to do this issue before the other docker-related issues. Github actions is already testing Shadow under Ubuntu 18.04. Setting up docker with Ubuntu 18.04 first will allow us to identify and fix any issues that are specific to running under docker. Once we know we have docker working, we can then add other Linux distros and fix any bugs that are distro (but not docker) specific.