It seems this is missing functionality - compared to the official wordpress docker, which allows you to use specify both wordpress version and PHP version.
The work around I have for this is to locate the lastest tagged version of PHP 7.4 and to specify that particular version to build on docker. You can view all the versions here:
It seems that " 5.9.2-debian-10-r24" is the last known version to use PHP 7.4, before it forces everyone to run with PHP 8.
Once I loaded this image, I then update Wordpress to the latest version inside the wp-admin panel.
Hope this helps someone else out.
It would be great if bitnami also builds it in a way that you can specify which version of PHP more easily.