v9.0.4

v9.0.4 (#267)

Minor updates
* Using Nginx v7.3.0 base images - including Alpine S6 v6.6.0, BusyBox 1.37.0, Nushell build 261014, zstd compression on Alpine 3.20 and above, and bf-healthcheck
* Using PHP scripts from PHP v3.1.20, including fixes to the ext and ini modules
* Using symbolic links for php and php-fpm instead of hard links

Development updates
* Adding tests for PHP-FPM, loading the environment and overriding configuration
* Replacing test.sh with test.nu, which can test every PHP version at once
* Updating dev tags to match publish tags with a '-dev' suffix
* Removing a stray .DS_Store file

Documentation updates
* Rewriting README
* Removing reference to the Apache image
* Adding guidance for working across the ecosystem