docker/lookaside.go
func signatureStorageURL(base signatureStorageBase, manifestDigest digest.Digest, index int) *url.URL {
	if base == nil {
		return nil
	}
	url := *base
	url.Path = fmt.Sprintf("%s@%s=%s/signature-%d", url.Path, manifestDigest.Algorithm(), manifestDigest.Hex(), index+1)
	return &url
}
func signatureStorageURL(base signatureStorageBase, manifestDigest digest.Digest, index int) *url.URL {
	if base == nil {
		return nil
	}
	url := *base
	url.Path = fmt.Sprintf("%s@%s=%s/signature-%d", url.Path, manifestDigest.Algorithm(), manifestDigest.Hex(), index+1)
	return &url
}
// signatureStorageURL returns an URL usable for acessing signature index in base with known manifestDigest, or nil if not applicable.
// Returns nil iff base == nil.
// NOTE: Keep this in sync with docs/signature-protocols.md!