Add demos with plumber and Shiny

[nuest]

This might be the most realistic use case: an organisation is running Windows containers on a Server (and you cannot run Linux containers in Windows at the same time) and they want to run an R-based app accessible over HTTP.

• plumber (extra Dockerfile including plumber, then mount existing example scripts from the package)
• ~RStudio Server~~ (only for Linux)
• Shiny (extra Dockerfile installing Shiny and running an example app included in the package)

[nuest]

The string execution of Windows Containers really gave me a hard time here, because it seems they do not distinguish really well between " and '. This seems to be my lack of knowledge of using R on Windows:

PS C:\Users\d_nues01> R.exe -e "library('shiny'); runExample('01_hello')"
Der Befehl "C:\PROGRA~1\R\R-34~1.3\bin\x64\R.exe" -e "library" ist entweder falsch geschrieben oder
konnte nicht gefunden werden.

The quirks of powershell escaping might be the root problem: https://stackoverflow.com/questions/18116186/escaping-quotes-and-double-quotes

Solved by using Rscript.exe, more than one -e, and _not putting spaces around ,port=1234. No idea which one did the trick.