syntax for overriding userdb fields from passdb config

[Thomas Möhle]

Dear Dovecot developers,

how exactly is the syntax to override a userdb field within the passdb
configuration? I cannot find any documentation on this in the wiki.

Let's say, I'd like to override the userdb extra field "chroot" with
/var/mail. How exactly would be the notation?

My auth-sql.conf.ext looks like this:

passdb {
  driver = sql
  args = /etc/dovecot/dovecot-sql.conf.ext
}
userdb {
  driver = sql
  args = /etc/dovecot/dovecot-sql.conf.ext
}

I already tried these modifications within the passdb area:
override_fields = userdb_chroot=/var/mail
override_fields = userdb:chroot=/var/mail
override_fields = %{userdb:chroot}=/var/mail
override_fields = %{userdb_chroot}=/var/mail

but none of the above returned /var/mail when I tried to hand
%{userdb:chroot} to an external script.

Thanks for your help,
Thomas Möhle