Docker supports passing in environment variables to your containers as a handy way to easily switch environments when using multiple docker-compose files. For example you may have a base docker-compose.yml with a docker-compose.dev.yml and docker-compose.prod.yml file that specify environment variables for database hostnames/credentials.
The issue is simply accessing these environment variables directly isn't possible in openresty (e.g. using os.getenv()).
The issue is caused by NGINX blocking all environment variables by default. To make them accessible again you need to re-define them in your nginx.conf, but with no values, e.g:
You should then be able to access them as normal.