Fix references to docker registry image and fix docs

buildrunner/docker/multiplatform_image_builder.py

@@ -193,7 +193,7 @@ def _start_local_registry(self):
         """
         if not self._local_registry_is_running:
             logger.debug("Starting local docker registry")
-            container = docker.run("registry", detach=True, publish_all=True)
+            container = docker.run(f"{self.docker_registry}/registry", detach=True, publish_all=True)
             ports = container.network_settings.ports
 
             # If any assert fails something changed in the registry image and we need to update this code
@@ -394,7 +394,7 @@ def build_multiple_images(
             tags (List[str], optional): The tags to apply to the image. Defaults to None.
             push (bool, optional): Whether to push the image to the registry. Defaults to True.
             do_multiprocessing (bool, optional): Whether to use multiprocessing to build the images. Defaults to True.
-            docker_registry (str, optional): The docker registry to push the image to. Defaults to None.
+            docker_registry (str, optional): The docker registry to pull images from. Defaults to None.
             build_args (dict, optional): The build args to pass to docker. Defaults to None.
             inject (dict, optional): The files to inject into the build context. Defaults to None.