ci: Switch to DOCKER_DRIVER=overlay because overlay2 seems to have problems
Currently the CI mostly fails because at the end of building the docker image we get en error
"creating overlay mount to /var/lib/docker/overlay2/<...>/merged: device or resource busy".
It seems to be a problem with a newer host kernel and the overlay2
driver, so switch to overlay
.
cf: See: https://github.com/docker/for-linux/issues/711