image/memory.go
func (i *memoryImage) Signatures(ctx context.Context) ([][]byte, error) {
	// Modifying an image invalidates signatures; a caller asking the updated image for signatures
	// is probably confused.
	return nil, errors.New("Internal error: Image.Signatures() is not supported for images modified in memory")
}
func (i *memoryImage) Signatures(ctx context.Context) ([][]byte, error) {
	// Modifying an image invalidates signatures; a caller asking the updated image for signatures
	// is probably confused.
	return nil, errors.New("Internal error: Image.Signatures() is not supported for images modified in memory")
}
// Signatures is like ImageSource.GetSignatures, but the result is cached; it is OK to call this however often you need.