Docker: build rest-server at container build time

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.

changelog/unreleased/issue-146

@@ -0,0 +1,5 @@
+Build rest-server at docker container build time
+
+Add a build stage such that the latest rest-server is always built and packaged.
+This is done in a standard golang container to ensure a clean build environment
+and only the final binary is shipped rather than the whole build environment.