[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