[Mail] Partially revert e0260299 and make Postfix listen to port 25 again

I have no idea why I removed port 25 from Docker host mappings. While removing 993 (IMAP over SSL) and 465 (SMTPS) because they never worked and we use STARTTLS on 587 and 143, I probably removed 25 by error. Or because I thought it would be useless (why ?).

And also :

• Move away ENV from Dockerfile (better to have only one self-documented place, ie Compose)
• Remove smtps service from master.cf as it is obsolete and not mapped to any host port
• Remove virtual_minimum_uid already set to the default and does not seem useful without virtual_uid_maps
• Remove unused files (old private key, old rsyslog config)
• Try to clarify some comments (not sure they are tho)
• Fix syntax for milter filters (from Romain's hotfix on monitoring)

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

-This directory is for storing private keys associated with DKIM signing with
-opendkim.
-
-Here is advice from upstream
-
-(4) Store the private key in a safe place.  We generally use a path like
-    /var/db/dkim/SELECTOR.key.pem (where "SELECTOR" is the name you chose).
-    The /var/db/dkim directory and the associated .pem file should be owned by
-    the user that will be executing the filter (preferably not the
-    superuser) and be mode 0700 and 0600 respectively.
-
-In Debian, we use /etc/dkimkeys by default and the directory permissions and
-ownership are set correctly.  Ensure that the private key is owned by the
-opendkim user and the permissions are 0600.