Proxy requests to /backend to the backend and to get rid of CORS

[roschaefer]

Is your feature request related to a problem? Please describe.
In the docker setup, http://localhost:3000 can be reached from the host system but not from the docker network. Additionally, backend url and frontend url are different, which requires us to setup CORS.

Describe the solution you'd like
It would be great to have one endpoint on the server side rendered frontend, e.g. '/backend that proxies to the backend. Thus, we would not have to use CORS, as the request goes to the same origin.