bug 1468491: Fix production mode instructions

"make build-static" will use
DJANGO_SETTINGS_MODULE=kuma.settings.local
by default, which disables hashed assets. The plain "docker" command is
needed to override the environment variable.

docs/development.rst

@@ -321,7 +321,7 @@ To emulate production, and test compressed and hashed assets locally:
 #. Set the environment variable ``DEBUG=False``
 #. Start (``docker-compose up -d``) or restart (``docker-compose restart``)
    your Docker services.
-#. Run ``docker-compose exec web make build-static``.
+#. Run ``docker exec -e DJANGO_SETTINGS_MODULE=kuma.settings.prod kuma_web_1 make build-static``.
 #. Restart the web process using ``docker-compose restart web``.
 
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