Using docker's multi-stage builds we can build the restic/rest-server
within a golang build environment then create a container for use
(without the build environment) in a second build stage.
The advantages are:
1. Building the rest-server is predictable in a pristine environment
each time.
2. Container builds ensure we get the latest rest-server every time.
Updated README with details of new docker build approach, and added
changelog for unreleased changes.
