tests & disclaimer

[bpmct]

As we plan to add more images, let's add both a) tests to ensure basic functionality does not break and b) a disclaimer on README that these are references and there is no compatibility guarantee so it may be best to hardcode SHAs or mirror specific versions.

[johnstcn]

In the past I've used goss to peform basic docker image validations. We don't have to include it in the image, but it's a small self-contained binary so it's not a huge deal if we do.