document base image choice #223

as of now, there still appears to be no publicly available docker image from a reputable source which:

• has R installed
• is based on ubuntu (for familiarity) (r-base can't do it)
• can pinpoint R version (and linux) via tags (r-hub can't do it)
• offers multi-platform support (rstudio/rbase can't do it)

... soo we're stuck with ubuntu and adding R by hand