docs: add docker tips

[cdaringe]

Problem

It's common to install node in docker, and using NVM is quite helpful in this regard. Installing and using NVM isn't always. straightforward.

Solution

Make it easier for users to setup nvm in docker by droppin' some docs.

Ref: https://stackoverflow.com/questions/25899912/how-to-install-nvm-in-docker

[cdaringe]

@ljharb , I'm open to your suggestion, but those docker files fundamentally are for a different intent. Those are for running a first-class NVM image, where the above instructions teach the user how to make any image a NVM image. Implicitly, those docker files do the required work, but they do so with noise interspersed, and without a clear focus on the requirements to make NVM work in any image. Perhaps simply sprinkling on some extra verbage with reference to those would be useful? I don't think references to those alone are the correct solve.