chore(python): detect when samples testing requires custom dockerfile (#1277)

* chore(python): automatically detect when samples testing requires custom dockerfile

* address review feedback

* lint

Author: parthea

synthtool/gcp/common.py

@@ -266,9 +266,11 @@ def py_library(self, **kwargs) -> Path:
         if Path("google/cloud").exists():
             kwargs["is_google_cloud_api"] = True
 
-        # Assume the python-docs-samples Dockerfile is used for samples by default
-        if "custom_samples_dockerfile" not in kwargs:
-            kwargs["custom_samples_dockerfile"] = False
+        # If Dockerfile exists in .kokoro/docker/samples, add kwargs to
+        # signal that a custom docker image should be used when testing samples.
+        kwargs["custom_samples_dockerfile"] = Path(
+            ".kokoro/docker/samples/Dockerfile"
+        ).exists()
 
         ret = self._generic_library("python_library", **kwargs)