Hey @martti
I think this thread includes a solution for you: Partial site hosting with reverse proxy (tl;dr, you need to make sure to modify the host header in nginx when you do the proxy pass)
One thing to note, proxying a webflow subdomain is not supported and will display webflow branding on the site. If you add a custom domain and reverse proxy that you will not see any branding.
Let me know if you have any other questions!