Nginx proxying Nodejs (Dokku). CORS response headers not passing through

I'm using Dokku to host my app at DigitalOcean. Dokku run nginx 1.6 to proxy Docker apps simulating a Heroku-like environment. The app's all share similar default configs like below.

My Node.js server uses CORS middleware to tell the browser to allow www.myapp.com to make calls to api.myapp.com:

This works fine on my local computer. When I deploy it, I'm getting a CORS error in the browser:

XMLHttpRequest cannot load https://api.myapp.com/r_u_up. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'https://www.myapp.com' is therefore not allowed access. The response had HTTP status code 502.

So, WTF, over.

I found this nginx CORS config but it seems very crufty. Is this old code, or the best way? This plugin uses that config.

I'd prefer a simpler config that just passes the response headers through. My app doesn't need nginx to intercept them. How can I configure that?

App nginx.conf's:

upstream www { server 172.17.0.135:5000; }
server {
  listen      [::]:80;
  listen      80;
  server_name www.myapp.com ;
  return 301 https://www.myapp.com$request_uri;
}

server {
  listen      [::]:443 ssl spdy;
  listen      443 ssl spdy;
  server_name www.myapp.com;


  keepalive_timeout   70;
  add_header          Alternate-Protocol  443:npn-spdy/2;
  location    / {
    proxy_pass  http://www;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection upgrade;
    proxy_set_header Host $http_host;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header X-Forwarded-For $remote_addr;
    proxy_set_header X-Forwarded-Port $server_port;
    proxy_set_header X-Request-Start $msec;
  }
  include /home/dokku/www/nginx.conf.d/*.conf;
}