.ebextensions with Docker on elasticbeanstalk

I am attempting to configure nginx on an elastic beanstalk Docker instance to allow the use of websockets. Using the information found here I added a .ebextensions/01_files.config to the zip I am deploying.

The zip contains .ebextensions/01_files.config, a Dockerfile, and a tgz of my application.

The content of 01_files.config is the following:

files:
    "/etc/nginx/conf.d/websocketupgrade.conf" :
        mode: "000755"
        owner: root
        group: root
        content: |
            proxy_set_header        Upgrade         $http_upgrade;
            proxy_set_header        Connection      "upgrade";

Elastic beanstalk seemingly ignores my .ebextensions. The file /etc/nginx/conf.d/websocketupgrade.conf is never written.

What is the correct way to use .ebextensions with a Docker container on elastic beanstalk?