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