win32: add rsync install in Dockerfile

This tool is needed during build when using make-ring.py script from
ring-project repo. It was included as base tool in previous Fedora 25
docker image but not anymore in Fedora 27.

Change-Id: I11833a5eb3cd3b7a0bcb705d4a136f349035dd45
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>