rechunker: automatically handle container storage

[cgwalters]

Our current docs suggest podman run -v /var/lib/containers/storage:/var/lib/containers/storage but for rootless we need ~/.local/share/containers/storage which is non-obvious and confusing to users.

In bootc-dev/bootc#919 I wrote some code to automatically do the bind mounts from inside the container which avoids this confusion for users - but we'd need to detect the "running rootless" vs "rootful" case inside the container which we can also do by parsing /run/.containerenv - bootc has code for this too.