Add an Rprofile.site to link to the CRAN mirror

[Gutschlhofer]

Currently, it's set to a MRAN snapshot mirror, leading to packages not being up-to-date or not updateable if you do not specify a specific repository.

root@b044aee5c5e2:/usr/local/lib/R/etc# cat Rprofile.site 
options(repos = c(CRAN = 'https://cran.rstudio.com/'), download.file.method = 'libcurl')
options(repos = c(CRAN='https://mran.microsoft.com/snapshot/2019-03-14'), download.file.method = 'libcurl')

Just change the default to CRAN (= remove the MRAN)