Check env var for setting driver in tests

graphdriver/driver.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"github.com/dotcloud/docker/archive"
 	"github.com/dotcloud/docker/utils"
+	"os"
 	"path"
 )
 
@@ -66,9 +67,15 @@ func New(root string) (Driver, error) {
 	var driver Driver
 	var lastError error
 
-	if DefaultDriver != "" {
-		return GetDriver(DefaultDriver, root)
+	for _, name := range []string{
+		os.Getenv("DOCKER_DRIVER"),
+		DefaultDriver,
+	} {
+		if name != "" {
+			return GetDriver(name, root)
+		}
 	}
+
 	// Check for priority drivers first
 	for _, name := range priority {
 		driver, lastError = GetDriver(name, root)