Use a Windows Nano Server image instead of Server Core for multi-arch testing
Use a Windows Nano Server image instead of Server Core for multi-arch testing

This image is about 100 MB instead of about 2 GB for the Server Core,
decreasing disk requirements and hopefully significantly speeeding up
integration tests.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

mtrmac/skopeo,mtrmac/skopeo,mtrmac/skopeo