[keyserver] Add rsync to Dockerfile

Summary:
We use rsync here, so we need to make sure it's in our Dockerfile.

This diff also organizes the Dockerfile into sections. Figured that change was too small to require its own diff.

Depends on D3847

Test Plan: tested that yarn rsync 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/D3848