Fix "/ is not a shared mount, this could cause issues or missing mounts with rootless containers"

In the logs from the pipglr container, this error message is spewed on nearly every podman operation. I believe it's mostly a harmless annoyance, but maybe not. In any case the error can be manually stopped with: $ podman exec -itu root pipglr mount --make-rshared /. That leads me to believe all that's needed is a oneshot (rootless) root systemd service that executes the mount command on pipglr container startup. A (perhaps simpler) alternative is to adjust the run label to (somehow) mark / as shared. Not quite sure how to do that at the moment.