I recently upgraded to image: "rstudio/r-base:4.0.0-bionic" and was surprised to see that, despite using repos = "https://packagemanager.rstudio.com/all/__linux__/bionic/279" all packages were being installed from source (this is as of 2020-05-11).
It took me a good while to figure out that the problem was that (the public-facing) RSPM didn't seem to have binaries for 4.0.0 as of this writing, and everything works fine with image: "rstudio/r-base:3.6.3-bionic".
I might have missed this, but it might be helpful to surface the availability of binaries in RSPM for different R versions (and packages), just in case other people are as dense as I am.
I eventually found the respective section in the docs:
To install binary packages, click the Binary button and select a Client OS to get the binary repository URL for your operating system. Binary packages are available on Linux and Windows for R 3.4, 3.5, and 3.6.
Perhaps surfacing this right in the UI at https://packagemanager.rstudio.com/ might be helpful, as I'm guessing this is kind of a dynamic thing and RSPM binaries may lag behind R releases in future as well.
Falling back to installing from source is of course correct CRAN-y behavior, though in CI/CD settings it's often a headache with missing compile-time system dependencies and all that jazz.
I'd rather always wait until RSPM has the binaries before I jump to a new R release and it'd be great to have that info in a central impossible-to-miss spot.