[TC-27]: Add gosu to php images

Jira URL: https://jira.team23.de/projects/TC/issues/TC-27

💬 Description

This installs 'gosu' into our 'magento2-php' images like described on the install-page of 'gpsu' (https://github.com/tianon/gosu/blob/master/INSTALL.md)

Meta

• My Merge Request is named properly > [TODO] Issue summary
• WIP: / Draft: is resolved
• Replaced TODO for Jira URL