Use debian:stretch-slim as base image

[mmerian]

trafficstars

Hi !

This pull request changes the base image to debian:stretch-slim, in order to make the final image smaller.

Do you see any issues in using debian instead of ubuntu ?

[JasonRivers]

I will check this, In honesty the ubuntu vs debian isn't going to make a huge difference to the final image size as I currently (intentionally) leave the source in the image for nagios & plugins, I may change this, it was useful when building extra plugins, and fixing issues on-the-fly, but it's probably not required in the shipped image, this will reduce the image size significantly