Feature request: Implement Composer and Yarn on the build

[mmikhan]

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.