"bids-validator" docker imager in "GitHub packages" out of date / undocumented

[sappelhoff]

https://github.com/orgs/bids-standard/packages?repo_name=bids-validator

Maybe we should automate uploading the image upon release ... and document this as an alternative to DockerHub.

docs: https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry

[sappelhoff]

related to this, note that the pushes to "singularity" are broken, see: https://github.com/bids-standard/bids-validator/actions/workflows/docker-build-push.yml

This part: https://github.com/bids-standard/bids-validator/blob/880b27d5367625707daa47dc8fe190396d0633ac/.github/workflows/docker-build-push.yml#L36