Add Discovery Server reverse proxy guide for Caddy

[Corey-Keller]

The reverse proxy section of Syncthing Discovery Server has no guide on setting it up with Caddy (v2). I can't for the life of me figure out how to make it work.

[mcebular]

Hi Corey,

as you might have seen in the caddy issue caddyserver/caddy#3767, reverse proxying with caddy is, at the time I'm writing this, NOT possible due to a missing feature for passing a client certificate via a header (see linked issue).

And as mentioned in the linked issue aswell, I solved the issue myself by adding the missing feature (see commit mcebular/caddy@df63c2d) and building the caddy binary myself.

With my own caddy binary, my Caddyfile configuration looks as follows:

mydiscosrv.example.com {
	reverse_proxy 127.0.0.1:8443 {
		header_up X-Forwarded-For {http.request.remote.host}
		header_up X-Client-Port {http.request.remote.port}
		header_up X-SSL-Cert {http.request.tls.client.certificate_pem_escaped} # this is not (yet) supported in caddy v2!
	}

	tls {
		client_auth {
			mode request
		}
	}
}

[aoxiangtianji]

It's fixed now.
caddyserver/caddy#4241

[MrOggy85]

I have added a PR for the docs here: #812