Move PHP multi-dockerfile to multi-stage build

[BretFisher]

Now that all modern docker versions have multi-stage support, we should migrate the two dockerfiles we currently have to a multi-stage setup:

https://laravel-news.com/multi-stage-docker-builds-for-laravel

[vctls]

Interesting. This guy uses php-apache for the backend. On the other hand, he uses the Composer image and does a COPY from later.
On Sylius-Standard, they just install Composer in the php image. But they use a separate nginx, and they copy the assets to it.