use as git subtree (preferred)

add gitlab-ci to your extension

git subtree add --prefix .Build/gitlab-ci https://git.slub-dresden.de/typo3/gitlab-ci.git main --squash

add a .gitlab-ci.yml to your extension root, with this content

include:
  - local: '.Build/gitlab-ci/extension-gitlab-ci.yml'

pull from remote

git subtree pull --prefix .Build/gitlab-ci https://git.slub-dresden.de/typo3/gitlab-ci.git main --squash

push from remote

git subtree push --prefix .Build/gitlab-ci https://git.slub-dresden.de/typo3/gitlab-ci.git main

use from remote repository

add a .gitlab-ci.yml to your extension root, with this content

include:
  - remote: 'https://git.slub-dresden.de/typo3/gitlab-ci/-/raw/main/extension-gitlab-ci.yml'

create a .gitignore in your extension root, with this content

composer.lock
/vendor
/public
.php_cs.cache
.php-cs-fixer.cache

use rector local

composer require --dev ssch/typo3-rector

run rector

./vendor/bin/rector process . --config .Build/gitlab-ci/typo3-rector/Extension.php

use php-cs-fixer local

composer require --dev friendsofphp/php-cs-fixer

run php-cs-fixer

./vendor/bin/php-cs-fixer fix --config .Build/gitlab-ci/php-cs-fixer/Extension.php

use typo3-typoscript-lint local

composer require --dev helmich/typo3-typoscript-lint

run typo3-typoscript-lint

vendor/bin/typoscript-lint -c .Build/gitlab-ci/typoscript-lint/Extension.yml --fail-on-warnings