Merge pull request #26 from thaJeztah/18.06-backport-fix_TestExternalGraphDriver_pull

[18.06] Fix flaky TestExternalGraphDriver/pull test
diff --git a/integration/plugin/graphdriver/external_test.go b/integration/plugin/graphdriver/external_test.go
index 3596056..e34e55e 100644
--- a/integration/plugin/graphdriver/external_test.go
+++ b/integration/plugin/graphdriver/external_test.go
@@ -394,12 +394,12 @@
 		defer d.Stop(t)
 		ctx := context.Background()
 
-		r, err := c.ImagePull(ctx, "busybox:latest", types.ImagePullOptions{})
+		r, err := c.ImagePull(ctx, "busybox:latest@sha256:bbc3a03235220b170ba48a157dd097dd1379299370e1ed99ce976df0355d24f0", types.ImagePullOptions{})
 		assert.NilError(t, err)
 		_, err = io.Copy(ioutil.Discard, r)
 		assert.NilError(t, err)
 
-		container.Run(t, ctx, c, container.WithImage("busybox:latest"))
+		container.Run(t, ctx, c, container.WithImage("busybox:latest@sha256:bbc3a03235220b170ba48a157dd097dd1379299370e1ed99ce976df0355d24f0"))
 	}
 }