Docker-based test server needs mdns to resolve .local while testing qt
network stack in CI. Otherwise the tests would not see announced
services.
Relates to yocto/meta-boot2qt 7608ebe44f65f4d525081cf9a5b12cfa05792a8f
Change-Id: If834b01668bcf844e475e863eb1337849c718270
Reviewed-by: Samuli Piippo <samuli.piippo@qt.io>