reverseproxy: caddyfile: Don't add port if upstream has placeholder (#3819)

* check if the host is a placeholder

* Update modules/caddyhttp/reverseproxy/caddyfile.go

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

modules/caddyhttp/reverseproxy/caddyfile.go

@@ -155,7 +155,9 @@ func (h *Handler) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 			if err != nil {
 				host = upstreamAddr
 			}
-			if port == "" {
+			// we can assume a port if only a hostname is specified, but use of a
+			// placeholder without a port likely means a port will be filled in
+			if port == "" && !strings.Contains(host, "{") {
 				port = "80"
 			}
 		}