[keyserver] Add nvm to Dockerfile

Summary:
Our build/run scripts for the keyserver rely on nvm, so we need it in the Dockerfile.

Trying to lock to a specific version here. Inspired by this StackOverflow and this article.

Depends on D3848

Test Plan: tested that yarn prod worked inside the Docker image

Reviewers: palys-swm, jimpo

Reviewed By: palys-swm

Subscribers: Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3849