There are a bunch of components such as incus or LXC that also use
`boot.isContainer`, so we'd have to differentiate between "OS container"
and "actually nspawn".
This became necessary for the file-systems part where nspawn takes care
of setting up special filesystems like `/proc`, `/dev` etc., but others
don't.
To allow for a `boot.isContainer` being less overloaded, this introduces
`boot.isNspawnContainer` that is exclusively used for nspawn-specific
things. When `true`, `boot.isContainer = true;` is implied.
a532cb052e