Add version of the `ml-base` Image that is built off of the Debian `stretch-slim` Docker image tag. This image is very similar to the Ubuntu image, given Ubuntu's Debian heritage, but the image is smaller as `stretch-slim` is 22 MB (slimmed down from Debian `stretch` at 45 MB) while Ubuntu `bionic` is 29 MB. The difference is small, but if there is space saving to be had it should be used for images like this.

As a result, add the Debian tags to the `README` and also make the `debian` image the recommended default image in the `README`.