title: Have nginx set header on response according to environment var
body: <p>I use a simple Nginx docker container to serve some static files. I deploy this container to different environments (e.g. staging, production etc) using Kubernetes which sets an environment variable on the container (let's say <code>FOO_COUNT</code>).</p>
  
  <p>What's the easiest way of getting Nginx to pass the value of <code>FOO_COUNT</code> as a header on every response without having to build a different container for each environment?</p>