diff --git a/repo2docker/docker.py b/repo2docker/docker.py
index 6d622bec..2cb96107 100644
--- a/repo2docker/docker.py
+++ b/repo2docker/docker.py
@@ -80,7 +80,7 @@ class DockerEngine(ContainerEngine):
         help="""
         Extra commandline arguments to pass to `docker buildx build` when building the image.
         """,
-        help=True
+        help=True,
     )
 
     def __init__(self, *, parent):