Merge pull request #3399 from yorkedork/fix/env-file-validation

Adds additional validation to 'env_vars_from_file'.

compose/config/environment.py

@@ -28,6 +28,8 @@ def env_vars_from_file(filename):
     """
     if not os.path.exists(filename):
         raise ConfigurationError("Couldn't find env file: %s" % filename)
+    elif not os.path.isfile(filename):
+        raise ConfigurationError("%s is not a file." % (filename))
     env = {}
     for line in codecs.open(filename, 'r', 'utf-8'):
         line = line.strip()