As @yoonix and @Dennis pointed out, my web server (apache2) is configured to only allow public access to certain folders on the server. /opt is not one of them, but /var/www is.
Rather than change Apache2 configuration, I moved the SuiteCRM folder into /var/www/. Then I was able to get the install.php file to load using this in my browser:
Do not connect a development environment to a production database. Too much risk of untested code causing problems. And too often test environments allow poor security practices. Ideally, developers do not have direct access to production, no ssh and no DB credentials.
Create empty testing databases. Import enough example data to do testing.
Two production ...
I suggest you to use docker with nginx to proxy all requests, and then two different containers one with apache the other with nginx as well.
There is a really nice guide in this post Manage a multiple websites server with Docker Compose, NGINX and auto SSL certificates.
service apache2 reload
Found the solution here - trust me it works: