properly use warning

Signed-off-by: Tim van Katwijk <[email protected]>

dockerfile_parse/parser.py

@@ -11,6 +11,7 @@
 import logging
 import os
 import re
+import warnings
 from contextlib import contextmanager
 from shlex import quote
 
@@ -867,7 +868,7 @@ def image_from(from_value):
     :param from_value: string like "image:tag" or "image:tag AS name"
     :return: tuple of the image and stage name, e.g. ("image:tag", None)
     """
-    DeprecationWarning("Use image_name_from instead.")
+    warnings.warn("Use image_name_from instead.", DeprecationWarning)
     image, name = image_name_from(from_value)
     return str(image) if image else None, name