Dockerfiles for R plumber APIs

These images are a stable and convenient base for R APIs.

plumber-base

Built on the most stable R image, rocker/r-rev
Contains plumber and a minimal tidyverse
Contains convenience (and IMO best practice) tools like logging and config

plumber-mysql

Built on plumber-base for robustness
Adds system libraries and R packages necessary to connect to MySQL databases
NOTE: relies on RMariaDB instead of the legacy RMySQL package!