How to find the PHP version that is used by the latest `phpdoc/phpdoc` docker image?
The Dockerfile provided in the Q&amp;A indicates that the image uses the latest php:7 as a base image. One could inspect the version as follows:    

docker run -it php:7 php -v


Result

PHP 7.3.11 (cli) (built: Oct 25 2019 02:24:51) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.11, Copyright (c) 1998-2018 Zend Technologies




How to use an older version of php in conjunction with this container?

It is possible to use a specific version by checking the tags of php on dockerhub. According to this list, 7.2.25 is currently the latest 7.2. If one changes the from of the Dockerfile to FROM php:7.2.25 and build the image again, then 7.2.25 instead 7.3.11 will be used inside the container.