Volume owner research

[focusaurus]

• It looks like https://github.com/just-containers/s6-overlay solves the volume owner problem in much the same general approach we do, but they read a configuration file for the directories. Interesting to study but at the moment I think fix-volumes.sh is better for us since it needs no explicit configuration. If, however, we hit issues and we think derived images will need to explicitly list their volume mounts, we might look at s6-overlay specifically their fix-attrs script