Start docker-tools container

gistfile1.txt

Start the container

docker run -it --rm -v $(pwd):/opt -w /opt joomlaprojects/docker-tools bash

Any command you execute from now on is runnig in the container

git clone --single-branch -b 4.0-dev https://github.com/joomla/joomla-cms.git
cd joomla-cms
composer install
npm i

Now you have a actual joomla-4.0 dev version of joomla with all files you need

Next step is to get the PR Data
git fetch origin pull/21362/head:local_pr_21362

and update all files
npm run update

Done you can now start testing (after you installed Joomla!)

ctrl + c to exit the container but you can also let it run, you might will update or test a different PR