Feature request: Implement Composer and Yarn on the build

It would be nice to have the Composer and Yarn come with the build itself. I know that it's not something that everyone would want but I typically try to do everything through Docker containers and I don't have Composer and Yarn installed locally on my machine. So, the build fails when I run ./plugins.sh as expected since I don't have either of them locally installed.

https://github.com/Yoast/plugin-development-docker/blob/385e5370e96caaeecb909d4b179ea543a7b93ed1/plugins.sh#L48
https://github.com/Yoast/plugin-development-docker/blob/385e5370e96caaeecb909d4b179ea543a7b93ed1/plugins.sh#L52

So, it would be really handy to have the Composer and Yarn in the build itself. When there's no local Composer and Yarn installed, it will use the Docker containers instead.

Beginne mit plugins.sh in den referenzierten Zeilen 48 und 52 und untersuche anschließend die Docker-Konfiguration des Repositorys, um zu verstehen, wie der Build ausgeführt wird. Die Änderung ist abgeschlossen, wenn ./plugins.sh Composer und Yarn aus Docker verwenden kann, wenn keines der beiden Tools lokal installiert ist, und dabei das bestehende Build-Verhalten beibehält.