CI docker improvements
This branch updates the CI configurations for this repo, and the CI template for plugin repos, to:
- Cache images (shared between all jobs of same type in this repo)
- Add a new test job that validates that the software has been successfully installed in the docker images
- In this repo, the test job just runs
herd-control-app --help
; in plugin repos, the test job runsherd-control-app /herd-config.yml --test devices
.
- In this repo, the test job just runs