Limit occurrences of creating an environment object.

.env file is always read from the project_dir

Signed-off-by: Joffrey F <joffrey@docker.com>

tests/helpers.py

@@ -13,4 +13,5 @@ def build_config(contents, **kwargs):
 def build_config_details(contents, working_dir='working_dir', filename='filename.yml'):
     return ConfigDetails(
         working_dir,
-        [ConfigFile(filename, contents)])
+        [ConfigFile(filename, contents)],
+    )