cmd/derper: add robots.txt to disallow all

Fixes #5565

Change-Id: I5626ec2116d9be451caef651dc301b7a82e35550
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/derper/derper.go

@@ -203,6 +203,9 @@ func main() {
 			io.WriteString(w, "<p>Debug info at <a href='/debug/'>/debug/</a>.</p>\n")
 		}
 	}))
+	mux.Handle("/robots.txt", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		io.WriteString(w, "User-agent: *\nDisallow: /\n")
+	}))
 	debug := tsweb.Debugger(mux)
 	debug.KV("TLS hostname", *hostname)
 	debug.KV("Mesh key", s.HasMeshKey())