graphs: fix render forwarding

src/routes/graphs.js

@@ -14,7 +14,9 @@ function getGraphs(req, res, next) {
     delete parsedUrl.query['access_token'];
     delete req.headers['authorization'];
     delete req.headers['cookies'];
-    req.url = url.format({ pathname: 'render', query: parsedUrl.query });
+
+    // 'graphite-web' is the URL_PREFIX in docker-graphite
+    req.url = url.format({ pathname: 'graphite-web/render', query: parsedUrl.query });
 
     // graphs may take very long to respond so we run into headers already sent issues quite often
     // nginx still has a request timeout which can deal with this then.