These docker images are built and published using Github Actions workflows. You can inspect the workflows themselves in .github/workflows/.
whmcs-php-fpm
This image is built on top of official php-fpm image found on Docker Hub. It takes care of all the dependencies required to run WHMCS, the included dependencies are based on official WHMCS documentation.
whmcs-nginx
This image is built on top of official nginx image found on Docker Hub. It currently only copies over nginx.conf and generates default.conf based on a template.
This allows us to pass variables down to template when container is started. Which makes configuration easier than having to edit the file itself manually.