Change: Build rest-server at docker container build time

The Dockerfile now includes 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.

https://github.com/restic/rest-server/issues/146
https://github.com/restic/rest-server/pull/145