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

data/Dockerfiles/dovecot/rspamd-pipe-ham

@@ -0,0 +1,4 @@
+#!/bin/bash
+/usr/bin/curl -s --data-binary @- http://rspamd:11334/learnham < /dev/stdin
+# Always return 0 to satisfy Dovecot...
+exit 0