Ruby missing from the Dockerfile

[Oxidro]

Hi

I am currently migrating our Nagios from local installation to containerized using this image from the Docker Hub,
and it seems we have many external Nagios plugins and scripts using Ruby.

As most of the Nagios external plugins are either Python, Perl, Shell and Ruby,
it would be nice if Ruby installation could be added to the Dockerfile and update the Docker Hub image, as I think many people will find useful as it is for me.

Thanks!