diff --git a/flytekit/core/python_customized_container_task.py b/flytekit/core/python_customized_container_task.py
index e47fe77bf0..27b54fe7d0 100644
--- a/flytekit/core/python_customized_container_task.py
+++ b/flytekit/core/python_customized_container_task.py
@@ -13,6 +13,7 @@
 from flytekit.core.shim_task import ExecutableTemplateShimTask, ShimTaskExecutor
 from flytekit.core.tracker import TrackedInstance
 from flytekit.core.utils import _get_container_definition, load_proto_from_file
+from flytekit.image_spec.image_spec import ImageSpec
 from flytekit.loggers import logger
 from flytekit.models import task as _task_model
 from flytekit.models.core import identifier as identifier_models
@@ -157,11 +158,18 @@ def get_command(self, settings: SerializationSettings) -> List[str]:
         ]
 
         return container_args
-
+    
+    def get_image(self, settings: SerializationSettings) -> str:
+        if settings.fast_serialization_settings is None or not settings.fast_serialization_settings.enabled:
+            if isinstance(self.container_image, ImageSpec):
+                # Set the source root for the image spec if it's non-fast registration
+                self.container_image.source_root = settings.source_root
+        return get_registerable_container_image(self.container_image, settings.image_config)
+    
     def get_container(self, settings: SerializationSettings) -> _task_model.Container:
         env = {**settings.env, **self.environment} if self.environment else settings.env
         return _get_container_definition(
-            image=get_registerable_container_image(self.container_image, settings.image_config),
+            image=self.get_image(settings),
             command=[],
             args=self.get_command(settings=settings),
             data_loading_config=None,