allow configuring of the docker storage driver on gce

cluster/gce/config-default.sh

@@ -270,6 +270,11 @@ if [[ -n "${DISABLE_DOCKER_LIVE_RESTORE:-}" ]]; then
   PROVIDER_VARS="${PROVIDER_VARS:-} DISABLE_DOCKER_LIVE_RESTORE"
 fi
 
+# Override default docker storage driver.
+if [[ -n "${DOCKER_STORAGE_DRIVER:-}" ]]; then
+  PROVIDER_VARS="${PROVIDER_VARS:-} DOCKER_STORAGE_DRIVER"
+fi
+
 # Override default GLBC image
 if [[ -n "${GCE_GLBC_IMAGE:-}" ]]; then
   PROVIDER_VARS="${PROVIDER_VARS:-} GCE_GLBC_IMAGE"

cluster/gce/config-test.sh

@@ -307,6 +307,11 @@ if [[ -n "${DISABLE_DOCKER_LIVE_RESTORE:-}" ]]; then
   PROVIDER_VARS="${PROVIDER_VARS:-} DISABLE_DOCKER_LIVE_RESTORE"
 fi
 
+# Override default docker storage driver.
+if [[ -n "${DOCKER_STORAGE_DRIVER:-}" ]]; then
+  PROVIDER_VARS="${PROVIDER_VARS:-} DOCKER_STORAGE_DRIVER"
+fi
+
 # Override default GLBC image
 if [[ -n "${GCE_GLBC_IMAGE:-}" ]]; then
   PROVIDER_VARS="${PROVIDER_VARS:-} GCE_GLBC_IMAGE"

cluster/gce/gci/configure-helper.sh

@@ -852,6 +852,12 @@ function assemble-docker-flags {
     docker_opts+=" --live-restore=false"
   fi
 
+  # Override docker storage driver if the environment variable is set
+
+  if [[ -n "${DOCKER_STORAGE_DRIVER:-}" ]]; then
+    docker_opts+=" --storage-driver=${DOCKER_STORAGE_DRIVER}"
+  fi
+
   echo "DOCKER_OPTS=\"${docker_opts} ${EXTRA_DOCKER_OPTS:-}\"" > /etc/default/docker
 
   if [[ "${use_net_plugin}" == "true" ]]; then