Remaining steps for IPv6 support

Most of the work for IPv6 has already been done; this adds support for using ip6tables, makes sure ipv6 support is enabled on the docker0 bridge, and skips the first IP even on IPv6 so we don't have to use a sub-range.
I think in future we'll likely want to use both IPv4 and IPv6 at the same time.  It's only called with (name, !ipv6, ipv6) at the moment.

Happy to fix either way.