signing: add docker invocation

Add docker invocation to image.py to trigger signing of artifacts.

BUG=b:295358776
TEST=unit-test

Change-Id: I2fb01319008fb8280e82c57ffcb6ed5598f94f11
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4877516
Tested-by: Benjamin Shai <bshai@google.com>
Commit-Queue: Greg Edelston <gredelston@google.com>
Reviewed-by: Greg Edelston <gredelston@google.com>
Auto-Submit: Benjamin Shai <bshai@google.com>
diff --git a/api/controller/image_unittest.py b/api/controller/image_unittest.py
index c93d08c..0da766a 100644
--- a/api/controller/image_unittest.py
+++ b/api/controller/image_unittest.py
@@ -797,7 +797,14 @@
 
     def testValidateOnly(self):
         """Check that a validate only call does not execute any logic."""
-        req = image_pb2.SignImageRequest()
+        req = image_pb2.SignImageRequest(
+            result_path=common_pb2.ResultPath(
+                path=common_pb2.Path(
+                    path="/path/to/outside",
+                    location=common_pb2.Path.OUTSIDE,
+                )
+            )
+        )
         resp = image_pb2.SignImageResponse()
         rc = image_controller.SignImage(req, resp, self.validate_only_config)
         self.assertEqual(rc, controller.RETURN_CODE_VALID_INPUT)