Improve test CI to actually build SRH and run that image

[hiett]

Right now just runs from latest Docker Hub image, so it only useful for the nightly tests against the JS package.

Ideally, the Dockerfile would be built, and that image used to run the test

[hiett]

Can just have a different job to build first and pass the image through: https://docs.docker.com/build/ci/github-actions/share-image-jobs/