Add sieve support

core/config/dovecot.lda

@@ -1,5 +1,5 @@
 protocol lda {
   hostname = mail.docker.container
   postmaster_address = postmaster@mail.docker.container
-  mail_plugin_dir = /usr/lib/dovecot/modules/lda
+  mail_plugins = $mail_plugins sieve
 }

core/config/dovecot.sieve

@@ -0,0 +1,4 @@
+plugin {
+    sieve_before = /vmail/sieve/spam-global.sieve
+    sieve = /vmail/%d/%n/sieve/scripts/active.sieve
+}