Merge pull request #2237 from dnephin/env_ci_storage_graph

Make storage driver configurable in CI

script/ci

@@ -11,7 +11,9 @@ set -ex
 docker version
 
 export DOCKER_VERSIONS=all
-export DOCKER_DAEMON_ARGS="--storage-driver=overlay"
+STORAGE_DRIVER=${STORAGE_DRIVER:-overlay}
+export DOCKER_DAEMON_ARGS="--storage-driver=$STORAGE_DRIVER"
+
 GIT_VOLUME="--volumes-from=$(hostname)"
 . script/test-versions