[Enhancement] Add R version as a parameter to write_docker

[vinay-swamy]

Hi love the package, just one request - add r_version to write_dockerfile to allow user to specify the version of R they want to use, as opposed to using date to look up version. Useful if you don't know when a specific R version was published (/ to lazy to look it ups ;) ). I can make a pull request if you agree

[karthik]

Thanks @vinay-swamy! Appreciate the feedback. This is on the roadmap and I'll make sure to add this in the next update (which is overdue but there is now a conference deadline).

[ASBecker]

+1 for this. holepunch:::r_version_table is a bit outdated ;)

[karthik]

Thanks for the reminder @ASBecker I just pushed a fix.