fix: Remove VOLUME directive from Dockerfile

[jeboehm]

I'd like to suggest to remove both VOLUME directives from the non production build stages.
The VOLUME directive creates and mounts directories when creating a container. This is not always wanted, especially when using the image as a base.

See also:

• Reset properties inherited from parent image moby/moby#3465 (comment)
• Add an UNVOLUME or UNSET VOLUME statement to cancaled mount the default parent volume moby/buildkit#4802 (comment)
• mta and mda containers are creating and mounting volumes that are left uncleaned when the stack is shut down jeboehm/docker-mailserver#652

[schildbach]

I would rather not remove this, as it serves to persist the mail storage. Why would you not want to persist your mail?

You could base your image on the production-base target – it's without VOLUME.

[jeboehm]

Persistence itself is more or less established with volume mounts.
These are still possible in all container engines.

[schildbach]

The VOLUME declaration makes sure the volume is initialized with the right content and permissions.

[jeboehm]

The VOLUME declaration makes sure the volume is initialized with the right content and permissions.

What content?

I'd also like to point out that

docker/2.4.2/dovecot.conf

Line 18 in 698178f

mail_home = /srv/vmail/%{user | lower}

uses /srv/vmail as the path for mail storage, created in

docker/2.4.2/Dockerfile

Line 204 in 698178f

mkdir /srv/vmail -p && \

.
For some reason /srv/mail is used as the home directory, which is IMHO unnecessary. /srv/vmail is also documented in the README.

I've solved these inconsistencies in 58d65c0. I still think that one VOLUME statement in the production layer is best for both users and developers, so I've kept this.