Load a prepared session / workspace with RStudio Server

[MatthiasHinz]

Extends #37

Simple R images like rocker/r-ver do not come with a GUI, therefore, the use of R is restricted to the console.

With such an configuration (try for instance docker run -it --rm rocker/r-ver) users are not able to view R plots or any file or data that cannot be printed directly to the console.

Therefore, it would be beneficial to leverage Rstudio images (see rocker/rstudio) and thus restore sessions directly in an Rstudio Server session.

See https://github.com/rocker-org/rocker/wiki/Using-the-RStudio-image