Config file syntax: secrets from file in key=value

[Tobias Florek]

Hi!

I'd like to set up proxy authentication. This works fine, but I cannot 
get the passwords in  to be read from file.

Specifically I have this passdb block.

passdb {
   driver = passwd-file
   mechanisms = plain scram-sha-256
   default_fields = proxy=y host=dovecot-backend 
master=imapc-master-user proxy_mech=plain 
pass=</run/secrets/dovecot.imapc-master-user/password
   args = /run/secrets/dovecot.generated.app-passwd-file/passwd
}

This sets the password to the string 
/run/secrets/dovecot.imapc-master-user/password.  I'd like the password 
to be set to the content of said file.

Is it possible to get the password from a file or less desirable from 
the environment?  Reading 
https://doc.dovecot.org/configuration_manual/config_file/ I assume it's not.

Cheers,
  Tobias Florek