fix: default to empty list for docker build extra params metadata (aws#8941)

* fix: default to empty list for docker build extra params metadata

* fix: set default consistently for DockerBuildExtraParams

Author: reedham-aws

samcli/lib/build/app_builder.py

@@ -434,7 +434,7 @@ def _build_lambda_image(self, function_name: str, metadata: Dict, architecture:
         docker_tag = f"{image_name.lower()}:{tag}"
         docker_build_target = metadata.get("DockerBuildTarget", None)
         docker_build_args = metadata.get("DockerBuildArgs", {})
-        docker_build_extra_params = metadata.get("DockerBuildExtraParams", None)
+        docker_build_extra_params = metadata.get("DockerBuildExtraParams", [])
 
         if not dockerfile or not docker_context:
             raise DockerBuildFailed("Docker file or Docker context metadata are missed.")

samcli/lib/samlib/resource_metadata_normalizer.py

@@ -191,7 +191,7 @@ def _extract_image_asset_metadata(metadata):
             SAM_METADATA_DOCKERFILE_KEY: str(dockerfile_path.as_posix()),
             SAM_METADATA_DOCKER_CONTEXT_KEY: str(asset_path),
             SAM_METADATA_DOCKER_BUILD_ARGS_KEY: metadata.get(ASSET_DOCKERFILE_BUILD_ARGS_KEY, {}),
-            SAM_METADATA_DOCKER_BUILD_EXTRA_PARAMS_KEY: metadata.get(ASSET_DOCKER_BUILD_EXTRA_PARAMS_KEY, None),
+            SAM_METADATA_DOCKER_BUILD_EXTRA_PARAMS_KEY: metadata.get(ASSET_DOCKER_BUILD_EXTRA_PARAMS_KEY, []),
         }
 
     @staticmethod