Remove getattr usage

repo2docker/app.py

@@ -745,11 +745,9 @@ class Repo2Docker(Application):
                     picked_buildpack = self.default_buildpack()
 
                 picked_buildpack.appendix = self.appendix
-                picked_buildpack.labels = {
+                picked_buildpack.labels['repo2docker.version'] = self.version
-                    'repo2docker.version': self.version,
+                picked_buildpack.labels['repo2docker.repo'] = self.repo
-                    'repo2docker.repo': self.repo,
+                picked_buildpack.labels['repo2docker.ref'] = self.ref
-                    'repo2docker.ref': self.ref,
-                }
 
                 self.log.debug(picked_buildpack.render(),
                                extra=dict(phase='building'))

repo2docker/buildpacks/base.py

@@ -171,6 +171,7 @@ class BuildPack:
     def __init__(self):
         self.log = logging.getLogger('repo2docker')
         self.appendix = ''
+        self.labels = {}
         if sys.platform.startswith('win'):
             self.log.warning("Windows environment detected. Note that Windows "
                              "support is experimental in repo2docker.")
@@ -246,7 +247,7 @@ class BuildPack:
         """
         Docker labels to set on the built image.
         """
-        return getattr(self, 'labels', {})
+        return self.labels
 
     def get_build_script_files(self):
         """