dovecot: disable anvil authentication penalty

fix #441

cmdeploy/src/cmdeploy/dovecot/dovecot.conf.j2

@@ -194,6 +194,15 @@ service imap-login {
     process_min_avail = 10
 }
 
+service anvil {
+    # We are disabling anvil penalty on failed login attempts
+    # because it can only detect brute forcing by IP address
+    # not by username. As the correct IP address is not handed
+    # to dovecot anyway, it is more of hindrance than of use.
+    # See <https://www.dovecot.org/list/dovecot/2012-May/135485.html> for details.
+    unix_listener anvil-auth-penalty { mode = 0 }
+}
+
 ssl = required
 ssl_cert = </var/lib/acme/live/{{ config.mail_domain }}/fullchain
 ssl_key = </var/lib/acme/live/{{ config.mail_domain }}/privkey