Accessing environment variables in AWS Beanstalk ebextensions

I am trying to access an environment variable that I have defined in the AWS Beanstalk configuration. I need to access it within a config file in .ebextensions or in a file that is copied in place in a config file. I have tried the following:

container_commands:
  update_nginx_config:
    command: "cp .ebextensions/files/nginx/nginx.conf /etc/nginx/nginx.conf"

And in my nginx.conf file, I have tried to access $MYVAR, ${MYVAR} and {$MYVAR}, some of which was suggested here and here (the latter being directly within a config file).

files:
  "/etc/nginx/nginx.conf" :
    mode: "000644"
    owner: root
    group: root
    content: |
      $MYVAR ${MYVAR} {$MYVAR}

This does not work either. In all cases, the variable names are just output such as $MYVAR, so Beanstalk does not recognize my variables. I found the below in the AWS documentation about container_commands:

They also have access to environment variables such as your AWS security credentials.

This is great, but it does not say how.

How can I access an environment variable with ebextensions, be it within a config file itself or in a separate file that is copied in place?

Thank you in advance!