create docker container

[zkamvar]

There have been calls for the creation of a docker container that folks can use for The Workbench so that they can use something similar to make docker.

The docker container should be relatively straightforward to implement and we can implement two flavours:

1. slim with just R, pandoc, and the necessary packages with room to expand for other packages. This would be used if people want to render without previews.
2. fully featured with RStudio, R, and pandoc so that folks can work to preview their lessons.

Both of these can be based on containers from the Rocker Project or the base image for the R-Universe project where we can set the MY_UNIVERSE environment variable.