stringIO always needs a string.

Signed-off-by: Tim van Katwijk <timvankatwijk@hotmail.com>

Author: tim-vk

dockerfile_parse/util.py

@@ -48,7 +48,7 @@ def __init__(self, s, args=None, envs=None):
         :param envs: dict, environment variables to use; if None, do not
             attempt substitution
         """
-        self.stream = StringIO(s)
+        self.stream = StringIO(str(s))
         self.args = args
         self.envs = envs