Allow deployment internal mail

[ikselven]

Work in progress to implement #1.

What's been done so far?

• mysql backend configuration for postfix is filled in using the MYSQL* environment variables
• SQL queries for mailbox_domains and mailbox_maps against the horde have been put into postfix configuration
• the horde_db container is intialized with a table for the domains
• one domain is inserted (hardcoded though, still needs a variable in .env file)
• entrypoint.sh of postfix image has been extended to expand the configs, see https://github.com/maintaina/containers/commit/9a403b700f89354489ca6b251eb4189e0af68dd7

[ralflang]

Looks promising. As for the user vs mail address problem, multiple solutions are valid:

• use $conf['imap']['maildomain] and fallback to $conf['problems']['maildomain']
• allow any combination of a user from users table and a domain from the virtualdomains table

Later on, mailbox management could be delegated to the vilma app (which is not currently in usable state)

[ikselven]

Temporary solution for the user vs. mail address problem:

• mysql_virtual_mailbox_maps.cf queries user_uid of table horde_users
• Horde users must be named username@domain, e.g. user1@horde.dev.local

[ralflang]

Is this still in do not merge state?