Add postfix-runtime volume for queue_directory/data_directory

[chrisblech]

Postfix's queue_directory (/var/spool/postfix) and data_directory (/var/lib/postfix) previously had no persistent storage, so the mail queue and TLS session caches were lost on every container recreate. Point both at a single postfix-runtime named volume instead, and have the entrypoint create the directory tree and fix ownership on every start via "postfix check" + "postfix set-permissions" (idempotent, so it's safe on both first run and subsequent restarts).

Should be merged before integrating #55 , so states of all clients (i.e. temp-whitelists) for postscreen survive container restarts, rebuilds, crashes.

[springcomp]

@chrisblech looks good.

What’s the story for existing users ?
is that as simple as a down / up and we’re good to go ?

[chrisblech]

@springcomp yes: this only adds persistence of spool/cache dirs. Entrypoint script takes care of creating these dirs if not existent and setting the right permissions.
There is no need for migration or other changes for existing users as far as I can see.