[services] [electron update server] Add a Dockerfile

Summary: For easy distribution we need to package the update server into a container like the rest of the services.

Test Plan: Check if the update server correctly runs inside docker

Reviewers: atul, jon, varun, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6355