The question he's addressing isn't what you can do, but what is commonly being done. You don't need RVM, you don't need Docker. Hell, you don't need Rails. But those are the idiomatic layers of the stack, for a non-trivial part of the Ruby development community.
Sorry for the confusion JVM = Java Virtual Machine, RVM = Ruby Virtual Machine, PVM = Python Virtual Machine. What I was trying to point out the layers of vitalization just to run an application. Docker is yet another layer on top of the operating system (a virtual machine running inside an operating system inside an operating system possible ect).
Ah, ok. I can see where you're coming from, but I don't really think of Docker as being an extra layer on top. It's more a way of rearranging the OS layer itself along a certain axis. The cognitive overhead is similar to a virtualisation layer though, so that point stands.
