Docker. How to get environment variable in nginx access_by_lua_file lua script

Question

I have nginx in a docker container, where FORWARD_LOG env variable has value 1.

In nginx.conf:

env FORWARD_LOG;

My server config:

server {

    listen 80 default_server;
    server_name nginx;

    location / {
        #resolvers
        #resolvers_template

        set_by_lua $forward_log 'return os.getenv("FORWARD_LOG")';

        proxy_pass $scheme://$host$request_uri;

        lua_code_cache on;
        access_by_lua_file /etc/nginx/domain.lua;

        allow all;
    }
}

Now I want to get this variable in domain.lua script. I tried different methods:

local FORWARD_LOG = ngx.var.forward_log; gives an empty result.
local FORWARD_LOG = $forward_log; gives unexpected symbol near '$'.
local FORWARD_LOG = os.getenv("FORWARD_LOG")'; - returns nil.

How can I do that ?