build(docker): forward server name to the PHP application

fixes #246

docker/production/web-server/nginx.conf

@@ -61,6 +61,7 @@ http {
         location / {
             fastcgi_param SCRIPT_FILENAME /opt/castopod/public/index.php;
             include fastcgi_params;
+	    fastcgi_param SERVER_NAME $host;
             fastcgi_index index.php;
             fastcgi_pass php-handler;
         }
@@ -69,6 +70,7 @@ http {
             try_files $uri =404;
             fastcgi_param SCRIPT_FILENAME /opt/castopod/public/$fastcgi_script_name;
             include fastcgi_params;
+	    fastcgi_param SERVER_NAME $host;
             fastcgi_index index.php;
             fastcgi_pass php-handler;
         }