Exim4 relay rejected for ipv6 host

[MikeDombo]

https://github.com/tianon/dockerfiles/blob/master/exim4/Dockerfile#L29
^ Code seems to imply that only ipv4 hosts will be allowed to relay, though in the same file, it is listening on both ipv4 and ipv6.

[tianon]

Good catch -- Docker networks don't typically have IPv6 enabled, so this doesn't really make much difference for any of the ways I deploy this, but is probably something worth fixing.