provide an R kernel in the Lab

Based on this gitlab ticket and relevant discussions we had with Phase 2 component owners
we have this concrete requirement that a JupyterLab kernel capable of running R scripts is needed.

Therefore, i suggest in the new docker image that is currently being prepared that a default R
kernel for JupyterLab is installed that will be started from the R version that is available in the base docker image.

To achieve this the following actions are needed:

In the docker file that creates the base JupyterLab image,
after the installation of the R package and the "jupyter client" tools,
as root user run the following commands to install a JupyterLab kernel that can run R notebooks:

R --slave -e "install.packages('IRkernel')"  
R --slave -e "IRkernel::installspec(name = 'ir363', displayname = 'R 3.6.3')"  

(the above are bash commands, and they would need to be modified for inclusion in dockerfile with "RUN ...command.....")