This can be configured in postfix main.conf as
virtual_transport = lmtp:unix:private/dovecot-lmtp or mailbox_transport = lmtp:unix:private/dovecot-lmtp
From the postfix howto guide
mailbox_transport_maps (default: empty)
Optional lookup tables with per-recipient message delivery transports to use for local(8) mailbox delivery, whether or not the recipients are found in the UNIX passwd database.
The precedence of local(8) delivery features from high to low is: aliases, .forward files, mailbox_transport_maps, mailbox_transport, mailbox_command_maps,
mailbox_command, home_mailbox, mail_spool_directory, fallback_transport_maps, fallback_transport and luser_relay.
https://www.postfix.org/local.8.html
https://doc.dovecot.org/configuration_manual/howto/postfix_dovecot_lmtp/