Unset env vars behavior in 'run' mirroring engine

Unset env vars passed to `run` via command line options take the
value of the system's var with the same name.

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

compose/cli/main.py

@@ -891,7 +891,9 @@ def build_container_options(options, detach, command):
     }
 
     if options['-e']:
-        container_options['environment'] = parse_environment(options['-e'])
+        container_options['environment'] = Environment.from_command_line(
+            parse_environment(options['-e'])
+        )
 
     if options['--entrypoint']:
         container_options['entrypoint'] = options.get('--entrypoint')