add support for trustedProxy `GATEWAY` to determine automatically from gateway. That's useful in docker where an nginx on the host is not always on a known ip.

server.js

@@ -28,7 +28,12 @@ lasso.configure(require("./marko-config"))
 
 // Set trusted proxy
 if (config.trustedProxy != null) {
-  if (ip.isV4Format(config.trustedProxy) || ip.isV6Format(config.trustedProxy)) {
+  if (config.trustedProxy === "GATEWAY") {
+    const ip = require("child_process").execSync("/sbin/ip route|awk '/default/ { print $3 }'").toString().trim()
+    app.set('trust proxy', ip)
+
+    console.log("Trusting proxy from default gateway", ip);
+  } else if (ip.isV4Format(config.trustedProxy) || ip.isV6Format(config.trustedProxy)) {
     app.set('trust proxy', config.trustedProxy)
 
     console.log("Trusting proxy", config.trustedProxy);