That response means the key isn’t allowed to access the API you created.
It is unrelated to the network error you had earlier.
To test the proxying issue. Open a bash shell inside the docker container and try to call your API directly.
To test the authentication error (this one) then you should set your API to open so it doesn’t need a key, then when you make the request, you’ll see if your API is reachable by the proxy.