[keyserver] Dockerfile for Node keyserver

Summary:
With script dependencies in place, this diff completes the Dockerfile for the Node keyserver.

Depends on D3870

Test Plan: docker-compose up --build

Reviewers: palys-swm, jimpo

Reviewed By: palys-swm

Subscribers: Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3871