It seems that the RedHat registry does not add the header Docker-Content-Digest on the response when asking for a manifest which is what reg is checking.

https://github.com/genuinetools/reg/blob/master/registry/digest.go#L38

The digest is also in the manifest.

I had a go at changing the Digest method to use it from the manifest, which is quite elegant but I'm not sure how that accounts for version 1 manifests.

// Digest returns the digest for an image.
func (r *Registry) Digest(image Image) (digest.Digest, error) {
	if len(image.Digest) > 1 {
		// return early if we already have an image digest.
		return image.Digest, nil
	}

	mf, err := r.ManifestV2(image.Path, image.Reference())
	if err != nil {
		return "", err
	}

	return mf.Config.Digest, nil
}

The digest is also in the manifest.

I had a go at changing the Digest method to use it from the manifest, which is quite elegant but I'm not sure how that accounts for version 1 manifests.

// Digest returns the digest for an image.
func (r *Registry) Digest(image Image) (digest.Digest, error) {
	if len(image.Digest) > 1 {
		// return early if we already have an image digest.
		return image.Digest, nil
	}

	mf, err := r.ManifestV2(image.Path, image.Reference())
	if err != nil {
		return "", err
	}

	return mf.Config.Digest, nil
}

This is not the Digest but the Docker ID.