Inherit from workflow.pacta, or other base image

[AlexAxthelm]

In resolving #14, I've effectively re-implemented the Dockerfile from workflow.pacta, rather than inheriting from that image, as I was having issues installing packages (Outlined in r-lib/pak#638)

This issue is to return to inheriting from a base image that already has a lot of the work done for us, which should reduce build times and keep different versions in sync with each other.