code: def _get_build_path(app_spec):
      
      if os.path.isabs(app_spec['build']):
          return app_spec['build']
      return os.path.join(Repo(app_spec['repo']).local_path, app_spec['build'])
docs: Given a spec for an app, returns the value of the `build` field for docker-compose.
      If the path is relative, it is expanded and added to the path of the app's repo.