NGINX PHP-FPM (FastCGI) 504 Gateway Timeout error on

Increase PHP maximum execution time in /etc/php.ini:

max_execution_time = 300

Increase PHP-FPM request terminate timeout in the pool configuration (/etc/php/7.2/fpm/pool.d/www.conf): 

request_terminate_timeout = 300

Increase Nginx FastCGI read timeout  under the http section (/etc/nginx/nginx.conf or /etc/nginx/sites-available/nameofvirtualhost): 

fastcgi_read_timeout 300;

Screenshot of a website

PhantomJS Installation

Install PhantomJS on Ubuntu

sudo apt-get update
sudo apt-get install build-essential chrpath libssl-dev libxft-dev
sudo apt-get install libfreetype6 libfreetype6-dev libfontconfig1 libfontconfig1-dev

wget https://bitbucket.org/ariya/phantomjs/downloads/phantomjs-2.1.1-linux-x86_64.tar.bz2
tar xvjf phantomjs-2.1.1-linux-x86_64.tar.bz2 -C /usr/local/share/

sudo ln -sf /usr/local/share/phantomjs-2.1.1-linux-x86_64/bin/phantomjs /usr/local/bin

phantomjs --version
2.1.1

mkdir /path/to/screenshot
cd /path/to/screenshot

Copy rasterize.js in the directory where you want the resulting screenshot to take place.

wget https://raw.githubusercontent.com/ariya/phantomjs/master/examples/rasterize.js
phantomjs --debug=yes --ignore-ssl-errors=true --ssl-protocol=any --web-security=true rasterize.js https://enews.ssis.edu.vn enews.png "400px*300px" .25

The last variable is the zoom factor of the website.  In this case .25 means 25% of the website in a 400px by 300px thumbnail.