The idea is to have two separate servers, Apache running PHP 5.6 and MySql and Nginx with PHP 7 FPM. Read more about how to setup Nginx reverse proxy in this blog post.
The best way to get stated is to use the .example files in the repository
Use the nginx.conf.example file to setup the reverse proxy server.
Use the .example files from php-5-6-apache/public and php-5-6-apache/conf directories to setup Apache server with PHP 5.6 and add your website configuration and website source files.
Use the .example files from php-7/public and php-7/conf directories to setup PHP 7.0 FPM server and add website source files.
Use the .example file from nginx/conf directory to setup Nginx server with PHP 7.0 FPM.
From root, run docker-compose up -d --build. Docker will build the containers and copy contents of 'php-5-6-apache', 'php-7' and 'nginx' public and conf directories to the virtual machines /var/www/html and appropriate configuration direcories. Those files are also synced with the containers.
Grab a coffee, it can take a while.