pipeline: Test more of the stage steps implementation
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.