This morning I was tinkering a little and while installing a composer dependency I noticed the don't run composer as root message which led go down the rabbit hole a little
$ docker run -it --rm -u 1000 webdevops/php-apache:7.2 whoami
root
$ docker run -it --rm -u 1000 webdevops/php:7.2 whoami
root
$ docker run -it --rm -u 1000 webdevops/php-official:7.2 whoami
root
The expected behaviour here would be whoami: application I guess