Add testplan to TEST_CONTAINER_BUILD_SCRIPTS.

BUG=b:182898188
TEST=`call_scripts/test__build_test_service_containers`
TEST=CQ

Change-Id: Id547bbe55d1a76e673968502959645881a39485d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3335241
Reviewed-by: Tim Bain <tbain@google.com>
Tested-by: Andrew Lamb <andrewlamb@chromium.org>
Commit-Queue: Andrew Lamb <andrewlamb@chromium.org>
diff --git a/api/controller/test.py b/api/controller/test.py
index 5c031ed..e1cafe1 100644
--- a/api/controller/test.py
+++ b/api/controller/test.py
@@ -166,6 +166,11 @@
             TEST_SERVICE_DIR,
             'test_finder/docker/build-dockerimage.sh',
         ),
+    'cros-testplan':
+        os.path.join(
+            TEST_SERVICE_DIR,
+            'plan/docker/build-dockerimage.sh',
+        ),
 }