I'm want to use GitLab to setup CI code testing for PHP. It says in the documentation, https://docs.gitlab.com/runner/, that the shell executor is not secure, https://docs.gitlab.com/runner/executors/shell.html, but also states that SSH is vulnerable to MITM attacks, https://docs.gitlab.com/runner/executors/ssh.html. What is recommended to use as a GitLab runner if these options are not secure options? I want to test PHP code w/ PHPUnit, https://phpunit.de/, and it which the above two options seem the most straightforward. In other words, is Docker the most common or recommended way to setup a GitLab CI Runner?
I'm using Ubuntu 18.04 locally, and also RHEL for GitLab testing code in a PHP7.2 environment.