Make environment variables without a value the same as docker-cli.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

compose/config/config.py

@@ -827,7 +827,7 @@ def resolve_env_var(key, val):
     elif key in os.environ:
         return key, os.environ[key]
     else:
-        return key, ''
+        return key, None
 
 
 def env_vars_from_file(filename):

compose/container.py

@@ -134,7 +134,11 @@ class Container(object):
 
     @property
     def environment(self):
-        return dict(var.split("=", 1) for var in self.get('Config.Env') or [])
+        def parse_env(var):
+            if '=' in var:
+                return var.split("=", 1)
+            return var, None
+        return dict(parse_env(var) for var in self.get('Config.Env') or [])
 
     @property
     def exit_code(self):

compose/service.py

@@ -622,6 +622,8 @@ class Service(object):
             override_options,
             one_off=one_off)
 
+        container_options['environment'] = format_environment(
+            container_options['environment'])
         return container_options
 
     def _get_container_host_config(self, override_options, one_off=False):
@@ -1020,3 +1022,12 @@ def get_log_config(logging_dict):
         type=log_driver,
         config=log_options
     )
+
+
+# TODO: remove once fix is available in docker-py
+def format_environment(environment):
+    def format_env(key, value):
+        if value is None:
+            return key
+        return '{key}={value}'.format(key=key, value=value)
+    return [format_env(*item) for item in environment.items()]