And to add one more option to the comments (beyond [frontend] + php-fpm):

Nginx Unit https://unit.nginx.org/ - like Apache + mod_php runs as a single service, handles the multiprocessing of php (and other languages), static files, reverse proxy and even lets you configure both itself and php via a single configuration (either as a file or dynamically at runtime, via a socket): https://unit.nginx.org/configuration/#php

Here is an IRL config example: https://github.com/PrivateBin/docker-unit-alpine/blob/master...

The resulting container image can be pretty small: https://hub.docker.com/r/privatebin/unit-alpine