[Mail] Merge MTA and MDA, add doc, build via Compose and secrets

...l/pica-mail-mta/local_mailbox_unix_sasl/saslauthd-postfix

pica-mail/pica-mail-mta/minimal-postfix/Dockerfile

pica-mail/pica-mail-mta/opendkim-tables.sh

pica-mail/pica-mail-mta/rsyslog.conf

pica-mail/pica-mail-mta/sasl-test/Dockerfile

pica-mail/pica-mail-mta/sasl-test/README.md

pica-mail/pica-mail-mta/sasl-test/entrypoint2.sh

pica-mail/pica-mail-mta/sasl-test/ldap-virtual-mailbox-maps

pica-mail/pica-mail-mta/sasl-test/saslauthd-postfix

pica-mail/pica-mail-mta/saslauthd-postfix

pica-mail/pica-mail-mta/spam/dkimkeys/README.PrivateKeys

pica-mail/pica-mail-mta/spam/dkimkeys/rsakeys.table

pica-mail/pica-mail-mta/spam/dkimkeys/signingdomains.table

pica-mail/pica-mail-mta/spam/ignore.hosts

pica-mail/pica-mail-mta/spam/nov2018.picasoft.net.rsa

pica-mail/pica-mail-mta/spam/opendkim

pica-mail/pica-mail-mta/spam/opendkim.conf

pica-mail/pica-mail-mta/spam/opendmarc

pica-mail/pica-mail-mta/spam/opendmarc.conf

pica-mail/pica-mail-mta/test-auth_local_user/Dockerfile