Fix versions by default

[noamross]

I suggest that by default hole punch fix versions for R and packages and print an informative message for packages used. This would involve:

• Picking the appropriate Rocker base based on local R version.
• Using a runtime.txt to fix the MRAN date
• Identify non-CRAN packages and set appropriate fixed-version Remotes.

[karthik]

Right now it scans all the code and picks the version of R matching the last modified date of any file. That way, when someone enables binder on an old repo, we are not inadvertently forcing a newer version of R. So now it will automatically pick a rocker ver date based on file modification date. A user can overrride with by passing r_date.

Using a runtime.txt to fix the MRAN date

Once a Dockerifle is in place, runtime.txt is ignored. The MRAN date is fixed in the Dockerfile. Example

Identify non-CRAN packages and set appropriate fixed-version Remotes.

This is an open issue