Dovecot: make home dir distinct from mail dir (#3335)

* add new home dir for Dovecot

I tried changing the mail dir, but this is a _very_ disruptive change,
so I took approach 3 on
<https://doc.dovecot.org/configuration_manual/home_directories_for_virtual_users/>,
whereby the home directory is now inside the mail directory.

The MDBOX/SDBOX formats are not touched by this change. The change
itself could be considered breaking though.

* adjust Sieve tests accordingly

* Update target/dovecot/10-mail.conf

* Update target/dovecot/auth-passwdfile.inc

---------

Co-authored-by: Casper <casperklein@users.noreply.github.com>

target/dovecot/10-mail.conf

@@ -27,6 +27,7 @@
 #
 # <doc/wiki/MailLocation.txt>
 #
+mail_home = /var/mail/%d/%n/home/
 mail_location = maildir:/var/mail/%d/%n
 
 # If you need to set multiple mailbox locations or want to change default

target/dovecot/auth-master.inc

@@ -6,4 +6,3 @@ passdb {
   result_success = continue
   #auth_bind = yes
 }
-

target/dovecot/auth-passwdfile.inc

@@ -15,5 +15,5 @@ passdb {
 userdb {
   driver = passwd-file
   args = username_format=%u /etc/dovecot/userdb
-  default_fields = uid=docker gid=docker home=/var/mail/%d/%u
+  default_fields = uid=docker gid=docker home=/var/mail/%d/%u/home/
 }