catch ImageNotFound on initial pull

builder/app.py

@@ -100,6 +100,7 @@ class Builder(Application):
         client = docker.APIClient(version='auto', **kwargs_from_env())
 
         repo, tag = self.output_image_spec.split(':')
+        try:
             for line in client.pull(
                     repository=repo,
                     tag=tag,
@@ -110,6 +111,8 @@ class Builder(Application):
                     break
             else:
                 return
+        except docker.errors.ImageNotFound:
+            pass
 
         output_path = os.path.join(self.git_workdir, self.build_name)
         self.fetch(