Downgrade Docker 16.04 to upgrade llvm

[acpaquette]

Updates the dockerfile to build with an updated llvm package. Unfortunately this requires that the ubuntu container be downgraded to 16.04 because llvm stopped supporting 18.04 seemingly.

There is also the addition of a conda env into the image which isn't ideal but this was the easiest way to update the python version on 16.04

[CLAassistant]

All committers have signed the CLA.

[acpaquette]

Another potential solution would be to pin the llvm install to llvm 10, then pull the correct version of Angora to build against that. That would allow the container to remain at Ubuntu 18.04 and remove the anaconda environment from the build