https://bugs.winehq.org/show_bug.cgi?id=54818
--- Comment #2 from François Gouget fgouget@codeweavers.com --- It gets its configuration from the same DHCP server. So its "Primary DNS suffix" setting (and also "Connection-specific DNS Suffix" and "DNS Suffix earch List" in the ipconfig output) matches the "domain" and "search" lines of the Debian machine's /etc/resolv.conf file.
But to be sure there's no confusion, it's not part of a Windows domain.