I’m no longer with dgraph, so please don’t take my answer as canon/may be outdated (I’m responding since I’m tagged):
I don’t think most people really need to run ratel themselves, you can just use the version of ratel hosted at https://play.dgraph.io and point it at their local dgraph instance (by entering http://localhost:8080 on the web interface)
Since ratel is in browser Javascript, the api calls go directly from your browser to your dgraph instance (ie: it does NOT go via the cloud, or via dgraph servers).
My setup was to run dgraph with docker via the command here: https://dgraph.io/docs/get-started/ (you can add —platform linux/amd64 in order to suppress a warning, but it’s optional). And use the version of ratel at dgraph play.