Injecting os.environ in Environment instance happens outside of init method

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

compose/config/config.py

@@ -449,12 +449,12 @@ def process_config_file(config_file, environment, service_name=None):
 
 
 class ServiceExtendsResolver(object):
-    def __init__(self, service_config, config_file, environment=None, already_seen=None):
+    def __init__(self, service_config, config_file, environment, already_seen=None):
         self.service_config = service_config
         self.working_dir = service_config.working_dir
         self.already_seen = already_seen or []
         self.config_file = config_file
-        self.environment = environment or Environment()
+        self.environment = environment
 
     @property
     def signature(self):

compose/config/environment.py

@@ -41,19 +41,22 @@ class Environment(dict):
     def __init__(self, *args, **kwargs):
         super(Environment, self).__init__(*args, **kwargs)
         self.missing_keys = []
-        self.update(os.environ)
 
     @classmethod
     def from_env_file(cls, base_dir):
-        result = cls()
-        if base_dir is None:
+        def _initialize():
+            result = cls()
+            if base_dir is None:
+                return result
+            env_file_path = os.path.join(base_dir, '.env')
+            try:
+                return cls(env_vars_from_file(env_file_path))
+            except ConfigurationError:
+                pass
             return result
-        env_file_path = os.path.join(base_dir, '.env')
-        try:
-            return cls(env_vars_from_file(env_file_path))
-        except ConfigurationError:
-            pass
-        return result
+        instance = _initialize()
+        instance.update(os.environ)
+        return instance
 
     def __getitem__(self, key):
         try: