Function mount_ostree_state 

fn mount_ostree_state(
    root: &Dir,
    deployment: &Dir,
    var: &Dir,
    etc_transient: bool,
) -> Result<()>

Mount /etc and /var as ostree-prepare-root does at boot: /etc is the deployment’s persistent copy, or a transient overlay of /usr/etc when prepare-root.conf enables etc.transient.