fix: use format strings intead of os.path.join for Unix paths for Processing Jobs (aws#1435)

When using the Python SDK to construct paths for Docker containers,
no matter what the user's platform is, we still expect the
Docker container to use Ubuntu (since that's what all the
prebuilt SageMaker images use).

Author: laurenyu

src/sagemaker/processing.py

@@ -495,7 +495,7 @@ def _convert_code_and_add_to_inputs(self, inputs, s3_uri):
         """
         code_file_input = ProcessingInput(
             source=s3_uri,
-            destination=os.path.join(
+            destination="{}{}".format(
                 self._CODE_CONTAINER_BASE_PATH, self._CODE_CONTAINER_INPUT_NAME
             ),
             input_name=self._CODE_CONTAINER_INPUT_NAME,
@@ -508,7 +508,7 @@ def _set_entrypoint(self, command, user_script_name):
         Args:
             user_script_name (str): A filename with an extension.
         """
-        user_script_location = os.path.join(
+        user_script_location = "{}{}/{}".format(
             self._CODE_CONTAINER_BASE_PATH, self._CODE_CONTAINER_INPUT_NAME, user_script_name
         )
         self.entrypoint = command + [user_script_location]