nixos-container: Make show-ip work together with ipv4 + netmask

The containers local address can be given as ipv4 only or with a subnetmask in
CIDR notation in the container configuration, see [1]. This works fine but the
'nixos-container show-ip' only supports plain ipv4 addresses without the netmask
suffix.

Changed the regex to also match in case of a CIDR netmask suffix.

[1] https://github.com/NixOS/nixpkgs/blob/9939032e35bb144bc8f6b57ed6325089910f738c/nixos/modules/virtualisation/containers.nix#L382