How to: SPA-friendly mode
Some SPA applications counts on proxy to handle 404 on static asset in a special way, by redirecting it to "/index.html". This is similar to nginx's `try_files $uri $uri/ …` directive and, apparently, this functionality somewhat important for the modern web apps.

This mode is off by default and can be turned on by setting `--assets.spa` or `ASSETS_SPA=true` env.