Add hint for running Flowset Control behind a reverse proxy

Hi,

In order to correctly generate correct https URLs for the redirect_url in SSO (with Keycloak for example) instead of http you need to set the following env variable, so that spring correctly respects the reverse proxy:

```
     SERVER_FORWARD_HEADERS_STRATEGY: "framework"
```

Would be perhaps worth to add it as a hint in the docs.

Cheers,

Julian