[keyserver] Add Webpack builds to keyserver Dockerfile

Summary:
Totally forgot about this, and didn't notice it until I actually tried to load a webpage from the Node keyserver.

I basically forgot to run keyserver/bash/setup.sh, but some of that file isn't relevant and I think it helps readability / discoverability to explicitly put these commands in the Dockerfile.

Depends on D4171

Test Plan: Make sure that the crash that was appearing regarding an assets.json import goes away

Reviewers: palys-swm, atul

Reviewed By: atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4172