docker/lookaside.go
func (ns registryNamespace) signatureTopLevel(write bool) string {
	if write && ns.SigStoreStaging != "" {
		logrus.Debugf(`  Using %s`, ns.SigStoreStaging)
		return ns.SigStoreStaging
	}
	if ns.SigStore != "" {
		logrus.Debugf(`  Using %s`, ns.SigStore)
		return ns.SigStore
	}
	return ""
}
func (ns registryNamespace) signatureTopLevel(write bool) string {
	if write && ns.SigStoreStaging != "" {
		logrus.Debugf(`  Using %s`, ns.SigStoreStaging)
		return ns.SigStoreStaging
	}
	if ns.SigStore != "" {
		logrus.Debugf(`  Using %s`, ns.SigStore)
		return ns.SigStore
	}
	return ""
}
// ns.signatureTopLevel returns an URL string configured in ns for ref, for write access if “write”.
// or "" if nothing has been configured.