This PR provides some improvements for current milters integration inside container:

1. Upgrades milter_protocol from 2 to 6. It seems that there is no reason to keep it 2, because all apps that uses it (OpenDKIM, OpenDMARC) already supports 6 with their current versions.
2. Disables OpenDMARC milter for outgoing mails as far as it does no job. DMARC relies on SPF and DKIM and there is nothing to do with outgoing messages for it (opposite to OpenDKIM which must sign messages).
3. Disables duplicate milters checking when we receive mail back from AMaViS on 10025 Postfix port. Miltering before sending mail to AMaViS is enough.
4. Sets OpenDKIM and OpenDMARC to listen on more "traditional" ports 8891 and 8893. See blockquote description in this tutorial to find out why these numbers are meant to be more "traditional".
5. Configures OpenDMARC parameters AuthservID and TrustedAuthservIDs with HOSTNAME value which, accordingly to documentation, will automatically resolve to container hostname. So no need to inject those values in start-mailserver.sh script.
6. Relatively to previous one, moves creating of /etc/opendmarc/ignore.hosts file to Dockerfile instead of start-mailserver.sh.