Mostly to avoid polluting other logs and artifacts, and also to avoid
recompiling crosstests over and over. Eventually the artifacts produced
at this stage should be run on native Windows.
In order for this to work it is expected that the Docker host exposes
the devices in /dev/dri to the guest system, and that the render nodes
have GID 800 (usually that would be the "render" group, but the GID
for that group is dynamically assigned, so we explicitly agree on a
fixed number).
