#8259 Update docker & docs for newer Ubuntu LTS
Our 2 Dockerfiles and install_each_step.rst still mention Ubuntu 16.04, and 18.04 is the latest LTS. It'd be good to update to that. Might also help with [#8257]
Offhand I expect most things would "just work" but we should go through the steps and verify if any adjustments need to be made to the installation process
Done in https://forge-allura.apache.org/p/allura/git/merge-requests/322/