Don't force-add `/etc/dovecot` as a volume

[schildbach]

Admins usually don't configure configuration directories as a docker volume. Instead, they read-only-bind-mount individual config files directly – in my case just dovecot.conf. However this leads to an unused docker volume per container creation/start, slowly eating up disk space.

My proposal would be to not declare VOLUME /etc/dovecot in the Dockerfile.

[grooverdan]

agree on this.