[Postfix] Fix missing ltrim

data/Dockerfiles/postfix/syslog-ng.conf

@@ -20,6 +20,7 @@ destination d_redis_ui_log {
     persist-name("redis1")
     port(6379)
     command("LPUSH" "POSTFIX_MAILLOG" "$(format-json time=\"$S_UNIXTIME\" priority=\"$PRIORITY\" program=\"$PROGRAM\" message=\"$MESSAGE\")\n")
+    command("LTRIM" "POSTFIX_MAILLOG" "0" "10000")
   );
 };
 destination d_redis_f2b_channel {