For the R sample, where does the R runtime come from?

I'm trying to understand custom handlers, and how you write them in an arbitrary language.
R is a good example of this.
The R sample here though doesn't seem to tell me anything about where the R runtime actually comes from. The only Dockerfile provided uses a "node" base image.
Naively I was expecting there to be a Dockerfile for the R sample that specified some kind of base image that contained an R runtime environment? But I probably misunderstand how custom handlers, and custom image work.