Proxy connections to kinto-core

[nandiheath]

Is your feature request related to a problem? Please describe.
Right now if the dashboard is not publicly exposed, we need to do kubectl port-forward both dashboard and core for users accessing the dashboard

Describe the solution you'd like
We should modify the Dockerfile to make it as a nginx container, serving both the static content and a proxy to the core API

Additional context
Make sure the grpc-web stream works for the proxy. (Be aware of the nginx idle timeout)