@harrelfe I don't think the PPA requires a key.
One point of confusion is that the link I had sent you is somewhat outdated. Due to the changes introduced in the 3.5 release of R, the maintainers had to create a new PPA, c2d4u3.5, which is explained in this blog post.
The rocker project has successfully created Dockerfiles for all the recent Ubuntu releases using this PPA strategy. The image is called r-apt. Here is the relevant line:
add-apt-repository --enable-source --yes "ppa:marutter/c2d4u3.5"
Note that they also use ppa:marutter/rrutter3.5 in this Dockerfile, but again that is identical to the main source from CRAN, which you've already setup.
I ran a r-apt disco container (rocker/r-apt:disco) to confirm that you should currently list bionic for any distribution post 18.04:
$ cat /etc/apt/sources.list.d/marutter-ubuntu-c2d4u3_5-bionic.list
deb http://ppa.launchpad.net/marutter/c2d4u3.5/ubuntu bionic main
deb-src http://ppa.launchpad.net/marutter/c2d4u3.5/ubuntu bionic main