set snapshotted RSPM binaries as default CRAN mirror

[maxheld83]

expanding on #32 it might be helpful if the images shipped with the appropriate RSPM url, as in repos = "https://packagemanager.rstudio.com/all/__linux__/bionic/279" for Ubuntu Bionic, etc.

The rocker images have a neat practice where R releases get CRAN set to latest on the MRAN checkpoint server or to whatever latest snapshot was taken when, say, R 3.6.3 was still release.
This way, even the old images will, by default, "just work" and install the appropriate dependencies.

Though these kinds of black magic do of course increase the complexity.

[maxheld83]

sorry, maybe this belongs in rstudio/r-builds

[slopp]

We were trying to avoid too much magic here, especially since there are many folks using these R binaries for on-premise workflows where they actually need to set the repo to their Package Manager instance.

[maxheld83]

Thanks!

If anyone else stumbles upon this / might be interested in a container with snapshotted RSPM, I've logged a similar issue over at rocker-org/rocker#401