text: Title: Spurious comment in `Dockerfile`
  Body: The `Dockerfile` used by repo2docker when building a repository that doesn't contain a `postBuild` file contains a spurious line:
  ```
  # Make sure that postBuild scripts are marked executable before executing them
  ```
  towards the end of the file. You can reproduce this with ` repo2docker --debug --no-build https://github.com/binder-examples/requirements`.
  
  We should remove the comment when there isn't a `postBuild` file.