Update ADD handling in Dockerfiles and test bundle creation

blimp/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 
 # Convenience meta-target for all of Blimp's production & test code.
 group("blimp") {
@@ skipping 157 matching lines @@
     args = [
       "--build-dir",
       _rebased_out_dir,
       "--filelist",
       _rebased_dockerfile,
       _rebased_dockerfile_base,
       "--manifest",
       _rebased_blimp_engine_env_tests_manifest,
       "--output",
       rebase_path(_bundle),
+      "--tar-contents-rooted-in",
+      rebase_path("//")
     ]
   }
 }