如何动态设置ssl_证书的域

user www-data;
worker_processes auto;
pid /run/nginx.pid;

events {
    worker_connections 768;
}

env MYDOMAIN;

http {
    server {
        listen 80;
        listen 443 ssl;
        set_by_lua $MYDOMAIN 'return os.getenv("MYDOMAIN")';
        server_name $MYDOMAIN www.$MYDOMAIN;
        location / {
          proxy_pass http://127.0.0.1:5000;
          index  index.html index.htm;
        }
        ssl_certificate /etc/letsencrypt/live/$MYDOMAIN/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/$MYDOMAIN/privkey.pem;    
    }
}

我得到一个错误:

nginx: [emerg] BIO_new_file("/etc/letsencrypt/live/$MYDOMAIN/fullchain.pem") failed (SSL: error:02001002:system library:fopen:No such file or directory:fopen('/etc/letsencrypt/live/$MYDOMAIN/fullchain.pem','r') error:2006D080:BIO routines:BIO_new_file:no such file)

设置server_name可以正常工作,但是对于ssl_certificate和ssl_certificate_key,它实际上是取$mydomain的值。