Rebase Dovecot on Stretch slim, build from stable source with latest stable Pigeonhole for antispam replacement

data/Dockerfiles/dovecot/report-ham.sieve

@@ -0,0 +1,11 @@
+require ["vnd.dovecot.pipe", "copy", "imapsieve", "environment", "variables"];
+
+if environment :matches "imap.mailbox" "*" {
+  set "mailbox" "${1}";
+}
+
+if string "${mailbox}" "Trash" {
+  stop;
+}
+
+pipe :copy "rspamd-pipe-ham";