Jonathan,
Rocker should be building weekly r-devel and drd (similar, was meant to be a
littler "lighter", effectively the same) images.
As Merlise noticed, somehow the cronjob push to the Docker Hub build service
started to fail -- this sadly happens once in a while so I manually triggered
a rebuild. They should be good now for 'R 4.0.0 to be'. Remember to invoke it
as 'RD' (for R-Devel) in the image, there is also an r-release as just 'R'.
Your case of "am on mac and need newer stuff" was also encountered last year
by Roger Koenker whom I helped a little bit via a few short emails -- leading
him to go from "zero to hero" with Docker. We wrote a short blog post on
that which you can find at
http://dirk.eddelbuettel.com/blog/2019/08/05#023_rocker_debug_example
It's nice as it gives a full rundown of all steps.
Hope this helps, Dirk