Use header variable in NGINX to forward traffic

I am using NGINX (nginx Docker image) as a reverse proxy and want to use the $host variable in stream/server context to forward the traffic to specific host which is defined in the Host header parameter of the incoming request. The configuration is the following:

events {
}

stream {
  log_format log_stream '$remote_addr [$time_local] $protocol'
  '$status $bytes_sent $bytes_received $session_time';

  access_log /var/log/nginx/access.log log_stream;
  error_log  /var/log/nginx/error.log;

  server {
    resolver 8.8.8.8 ipv6=off;
    listen       127.0.0.1:18443;
    proxy_pass   $host:443;
  }
}

There is an error during starting:

[emerg] 1#1: unknown "host" variable

According to the nginx documentation this variable should be populated. Do you know how can i use it inside the server directive to forward the traffic? Can i use other header parameter with specified hostname/address to forward the traffic?

Your intention seems to be to pass through TLS connection via nginx stream module. If you want to target different destinations depending on the SNI field of TLS header, then you need to use the following configuration:

map $ssl_preread_server_name $destination {
    host1.example.com backend1;
    host2.example.com backend2;
    default backend3;
}

stream {
    upstream backend1 {
        server 192.168.100.1:443;
    }

    upstream backend2 {
        server 192.168.100.2:443;
    }

    upstream backend3 {
        server 192.168.100.3:443;
    }

    server {
        listen 127.0.0.1:443;
        proxy_pass $destination;
    }
}