only log Dockerfile at debug-level if not dry_run

avoids double-display when using --debug --no-build

Author: minrk

repo2docker/app.py

@@ -684,11 +684,12 @@ def build(self):
                 picked_buildpack.labels["repo2docker.repo"] = repo_label
                 picked_buildpack.labels["repo2docker.ref"] = self.ref
 
-                self.log.debug(picked_buildpack.render(), extra=dict(phase="building"))
-
                 if self.dry_run:
                     print(picked_buildpack.render())
                 else:
+                    self.log.debug(
+                        picked_buildpack.render(), extra=dict(phase="building")
+                    )
                     if self.user_id == 0:
                         raise ValueError(
                             "Root as the primary user in the image is not permitted."