Documentation: add example of persistent volume for Postfix queue

[lesinigo]

Following the current documentation the container will have ephemeral storage and could loose any unsent email if it gets stopped, removed and recreated.

Documentation should contain an example on how to keep Postfix queue on an externally provided volume to make it persistent.

[bokysan]

You are correct -- although the Dockerfile maps the /var/spool/postfix the documentation itself does not explicitly state this.

Merge requests are welcome :)

[lesinigo]

Yep I'm currently testing out your (really well made) container right now, so far without any volume mapped.
I will test volume mapping and queue persistence and send an MR for the docs as soon as time permits 👍

[Nuru]

If you set persistence.enabled: true then it will use a Persistent Volume to store the queue