diff --git a/nextstrain/cli/runner/docker.py b/nextstrain/cli/runner/docker.py
index 1a30315b..6b76dd2e 100644
--- a/nextstrain/cli/runner/docker.py
+++ b/nextstrain/cli/runner/docker.py
@@ -232,9 +232,10 @@ def test_image_version():
 
 def set_default_config() -> None:
     """
-    No-op.
+    Sets ``docker.image``, if it isn't already set, to the latest ``build-*``
+    image.
     """
-    pass
+    config.setdefault("docker", "image", latest_build_image(DEFAULT_IMAGE))
 
 
 def update() -> bool: