Implement basic sieve support using Dovecot.

The dovecot-sieve plugin is installed and configured to apply sieve
as soon as a .dovecot.sieve file is encountered in the virtual user's
home directory (that is /var/mail/${domain}/${username}/.dovecot.sieve).

Transport has been changed in the postfix configuration to use
Dovecot LDA (see http://wiki.dovecot.org/LDA/Postfix) to actually
enable sieve filtering.

Tests have been added.

test/tests.bats

@@ -129,9 +129,9 @@
 }
 
 @test "checking smtp: delivers mail to existing account" {
-  run docker exec mail /bin/sh -c "grep 'status=sent (delivered to maildir)' /var/log/mail/mail.log | wc -l"
+  run docker exec mail /bin/sh -c "grep 'status=sent (delivered via dovecot service)' /var/log/mail/mail.log | wc -l"
   [ "$status" -eq 0 ]
-  [ "$output" -eq 2 ]
+  [ "$output" -eq 3 ]
 }
 
 @test "checking smtp: delivers mail to existing alias" {
@@ -409,3 +409,13 @@
   [ "$status" -eq 0 ]
   [ "$output" = "my-domain.com" ]
 }
+
+#
+# sieve
+#
+
+@test "checking sieve: user1 should have received 1 in folder INBOX.spam" {
+  run docker exec mail /bin/sh -c "ls -A /var/mail/localhost.localdomain/user1/.INBOX.spam/new | wc -l"
+  [ "$status" -eq 0 ]
+  [ "$output" = 1 ]
+}