How to specify pattern for share folder using ACL vfile

[Adam Raszkiewicz]

My `90-acl.conf` file looks like:

    plugin {
      #acl = vfile:/etc/dovecot/global-acls:cache_secs=300
      #acl = mysql:/etc/dovecot/dovecot-dict-sql.conf.ext
      acl = vfile
    }

    # To let users LIST mailboxes shared by other users, Dovecot needs a
    # shared mailbox dictionary. For example:
    plugin {
      #acl_shared_dict = file:/var/lib/dovecot/shared-mailboxes
      acl_shared_dict = proxy::acl
    }


When ACLs were set for particular mailbox I can see that dovecot-acl file were 
created but I have no idea how to specify a pattern for shared folder like is 
in the `dovecot-dict-sql.conf.ext`

    map {
      pattern = shared/shared-boxes/user/$to/$from
      table = user_shares
      value_field = dummy

      fields {
        from_user = $from
        to_user = $to
      }
    }

My goal is to control how it appears on the client site. As for now is as 
`shared/user@domain/folder/` but I have no idea where it is defined.

Thanks,
Adam