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] 9939032e35/nixos/modules/virtualisation/containers.nix (L382)