Improve invalid container root errors in update-ldcache hook

[cdesiniotis]

The update-ldcache hook currently has one if statement to handle container root resolution errors and invalid root values (e.g. "" or /).

containerRootDir, err := s.GetContainerRoot()
if err != nil || containerRootDir == "" || containerRootDir == "/" {
	return fmt.Errorf("failed to determined container root: %v", err)
}

An empty root or / can therefore produce a message ending in <nil>. This one error branch should be split into two.

[efegokdemir]

I'd like to work on this. I'll reproduce the misleading nil root-resolution error on current main and add focused unit coverage for both lookup failures and invalid root values.