Added lowercase validation for image name argument

repo2docker/app.py

@@ -122,6 +122,17 @@ class Repo2Docker(Application):
                                extra=dict(phase='failed'))
                 sys.exit(1)
 
+    def validate_image_name(self, image_name):
+        """
+        Validate that the image_name read by argparse contains only lowercase characters
+        :param image_name: string argument read by argparse
+        :return: test and return image_name if it only contains lowercase characters else raise error
+        """
+        if not image_name.islower():
+            msg = "%r is not a valid docker image name. Image name can contain only lowercase characters." % image_name
+            raise argparse.ArgumentTypeError(msg)
+        return image_name
+
     def get_argparser(self):
         argparser = argparse.ArgumentParser()
         argparser.add_argument(
@@ -146,7 +157,8 @@ class Repo2Docker(Application):
         argparser.add_argument(
             '--image-name',
             help=('Name of image to be built. If unspecified will be '
-                  'autogenerated')
+                  'autogenerated'),
+            type=self.validate_image_name
         )
 
         argparser.add_argument(