Specify what kinda list extra_buildx_build_args can be

repo2docker/docker.py

@@ -75,7 +75,7 @@ class DockerEngine(ContainerEngine):
     )
 
     extra_buildx_build_args = List(
-        [],
+        Unicode(),
         help="""
         Extra commandline arguments to pass to `docker buildx build` when building the image.
         """,