Functionally testing pipeline stage steps (build, run, publish, deploy,
exports) is tricky on account of either having to mock all calls to
workflow script and/or pipeline runner methods, or somehow setup
dependent backend systems such as k8s for systems tests.

The lack of coverage for these methods has resulted in a number of found
bugs and incremental fixes that could have been found through testing,
so the value of heavy mocking and system-test complexity in these cases
seems to outweigh the cost.

By installing a stubbed version of the `docker-pusher` script into the
Jenkins container, we can run a system test for the `publish` step, and
by stubbing out some of the previous stage/step context, we can more
effectively test `PipelineStage` step methods.

Change-Id: Ibed4f781e6c46dcbf5741eaf8adb79ebe4ef2d39

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