9.x Dockerfiles still based on Ubuntu 18.04, but runtime has moved to 20.04

[Plug11]

All of the version 9.x dockerfiles are built on minimal which uses ubuntu:18.04 as its base image. However databricks runtime 9.x uses ubuntu:20.04

[williambrandler]

currently upgrading the glow docker container to Ubuntu 20.04, some changes were necessary to update R

projectglow/glow#452

#59

Not tested yet to see if this all works in Databricks, but the build worked fine