Remove "/var/lib/postfix" & "/var/mail" volumes

[tigerblue77]

• "/var/lib/postfix" : Useless IMHO, added to preserve TLS session cache ?
  => If so : replace volume with /var/lib/postfix/smtp_scache.db file directly ?
• "/var/mail" : Used to store local users' emails. In a relay/redirection configuration, this directory is not supposed to be used.

@wader feel free to let me know why you created these volumes if they are necessary :)

[wader]

I guess both was to preserve state between container updates and i wasn't sure where things might end up stored. I guess what needs to be preserved is opendkim and the queue?

queue_directory (default: see "postconf -d" output)
The location of the Postfix top-level queue directory. This is the root directory of Postfix daemon processes that run chrooted.

Hmm where would that be with this image?