Move to using posit package manager before July as MRAN becomes obsolete

[annakrystalli]

As of July MRAN will be obsolete.

Need to move to using posit package manager: https://packagemanager.rstudio.com/client/#/

Shouldn't be too much effort, probably just one line in the write_dockerfile function. Having said that, given packager manager does not take daily snapshot, I'm unsure how to handle dates where there is no snapshot.

[annakrystalli]

The setting of the repos option needs to change to options(repos='https://packagemanager.rstudio.com/cran/__linux__/focal/{date})' but some quick experimentation on binder showed that installation failed if there is no snapshot on the given date.

[zuphilip]

@annakrystalli Can we not replace this whole GitHub call in write_dockerfile with the following line?

RUN if [ -f install.R ]; then R --quiet -f install.R; fi

Thereby this will already use the RStudio Public Package Manager (for R>=4.0) and moreover (if I understand it correctly) the versions from a past snapshot of the package directory are used then.

Then no DESCRIPTION file might be needed but the file install.R can be used for any additional R package.