try to limit index cache max size

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

@@ -70,6 +70,12 @@ userdb {
 # Mailboxes are stored in the "mail" directory of the vmail user home.
 mail_location = maildir:{{ config.mailboxes_dir }}/%u
 
+# index/cache files are not very useful for chatmail relay operations 
+# but it's not clear how to disable them completely. 
+# According to https://doc.dovecot.org/2.3/settings/advanced/#core_setting-mail_cache_max_size
+# if the cache file becomes larger than the specified size, it is truncated by dovecot 
+mail_cache_max_size = 500K
+
 namespace inbox {
   inbox = yes