How can containered workspaces use MRAN/checkpoint or GRAN?

[nuest]

MRAN supports file style locations now: RevolutionAnalytics/checkpoint#218

Also, this issue RevolutionAnalytics/checkpoint#216 (comment) reports on how to make checkpoint work for an R Markdown document.

[MatthiasHinz]

We may also check GRAN for retrieving versions of packages
http://blog.revolutionanalytics.com/2014/08/gran-and-switchr-cant-send-you-back-in-time-but-they-can-send-r-sort-of.html

[nuest]

Since be default we rely on rocker/r-ver images, which use MRAN set to the image build date, this works already then.

But a specific MRAN-creation instruction would also be nice! Just provide a date and code similar to the one in r-ver-Dockerfile is created, see

• https://github.com/rocker-org/rocker-versioned/blob/master/r-ver/Makefile#L51
• https://github.com/rocker-org/rocker-versioned/blob/master/r-ver/Dockerfile#L113

The user would do something along the lines of...

setClass("CRANMirror", slots = list(baseurl = "string"), contains = "Instruction")
setClass("MRANMirror", slots = list(baseurl = "string", date = "date"), contains = "CRANMirror")

df <- dockerfile(...)
mirror <- CRANMirror("2017-01-01")

addInstruction(df, mirror)

[nuest]

The best way for now to select the MRAN mirror is to select the base image manually.

What containerit could do is to see if MRAN a configured repo in the current session, and then also use that.

See also #150