Read / write data with mounted volumes

[MatthiasHinz]

Similar to #51, this issue addresses that simple R/docker images can hardly display any plots or external files by themselves.

In addition, it is a common use-case to mount external volumes to docker containers (see e.g. https://github.com/rocker-org/rocker/wiki/Using-the-RStudio-image https://benmarwick.github.io/UW-eScience-docker-for-reproducible-research/#1)

There are at least to objectives to leverage mounted volumes with R:

1. The idea is that users may want to write data from a Docker container to a shared folder (e.g. a pdf, a RData file or any other output data) in order view / process / share analysis results on the host system.

2. Users may to use a mounted volume in order to replace the input of an analysis with their own, modified data.

Some open questions to be for addressed 1) and 2) are:

• How must the workspace be arranged?
• What happens when we mount the R working directory?
• (How) can we possibly replace / modify input data?
• (How) can we possibly redirect output data to the host?