Unit testing this library does not give adequate confidence in the
working state of the overall system. The provided `systemtest` Makefile
target can be used to run a very broad system-level test using Jenkins
running in a local Docker context. It's not safe to run in CI (requires
bind mounting the Docker socket within the Jenkins container so that it
can actual perform builds) but it's useful for local development.

Change-Id: Ib9645afa993fa7d247321f3307de30bcab627c20