ci: Switch to DOCKER_DRIVER=overlay because overlay2 seems to have problems

See:  https://github.com/docker/for-linux/issues/711

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>

ci/.gitlab-ci.yml

@@ -5,7 +5,7 @@ cache:
     - ccache
 
 variables:
-  DOCKER_DRIVER: overlay2
+  DOCKER_DRIVER: overlay
 # When running CI jobs, don't allow
 # HW backed renderes, due to the
 # variability it introduces.