Fixes issue reported here https://github.com/NixOS/nixpkgs/pull/515773#issuecomment-4501563586
Containers with privateNetwork have an eth0 interface configured
imperatively by the container setup script, so the networking-interfaces.nix module
doesn't know about it. Specifying the default gateway then fails
silently, unless this setup is mirrored in networking.interfaces.eth0
inside the container.
