docker: Update Dovecot config to 2.4

Debian 13, the new stable version, updated Dovecot from 2.3 to 2.4.
Unfortunately, the new Dovecot version introduced several changes to the
configuration file that are not backwards-compatible.

https://doc.dovecot.org/2.4.2/installation/upgrade/2.3-to-2.4.html

This patch updates our docker's Dovecot config to the new version.

Thanks to Ernesto Alfonso (erjoalgo@github) for reporting this problem
in https://github.com/albertito/chasquid/issues/80.

docker/dovecot.conf

@@ -1,4 +1,7 @@
 
+dovecot_config_version = 2.4.0
+dovecot_storage_version = 2.4.0
+
 #
 # Logging
 #
@@ -10,10 +13,11 @@ log_path = /data/dovecot/dovecot.log
 
 # Store emails in /data/mail/home/domain/user, which will be inside the
 # container's volume.
-mail_home = /data/mail/home/%d/%n
+mail_home = /data/mail/home/%{user|domain}/%{user|username}
 
 # Use Dovecot's native format.
-mail_location = mdbox:~/mdbox
+mail_driver = mdbox
+mail_path = ~/mdbox
 
 # User and group used to store and access mailboxes.
 mail_uid = dovecot
@@ -31,13 +35,13 @@ first_valid_gid = 100
 
 # Static file, in /data/dovecot/users.
 auth_mechanisms = plain
-passdb {
-  driver = passwd-file
-  args = scheme=CRYPT username_format=%u /data/dovecot/users
+passdb passwd-file {
+  passwd_file_path = /data/dovecot/users
+  auth_username_format = %{user}
 }
-userdb {
-  driver = passwd-file
-  args = /data/dovecot/users
+userdb passwd-file {
+  passwd_file_path = /data/dovecot/users
+  auth_username_format = %{user}
 }
 
 
@@ -102,8 +106,10 @@ service managesieve {
 }
 protocol sieve {
 }
-plugin {
-  sieve = file:~/sieve;active=~/.dovecot.sieve
+sieve_script personal {
+  driver = file
+  path = ~/sieve
+  active_path = ~/.dovecot.sieve
 }
 
 #
@@ -125,8 +131,6 @@ service auth {
 }
 service auth-worker {
 }
-dict {
-}
 service lmtp {
   # This is used by mda-lmtp.
   unix_listener lmtp {