You can use it as a Github Action like this:

.github/workflows/lint.yml

on: [push, pull_request]
name: Main
jobs:
  php-cs-fixer:
    name: PHP-CS-Fixer
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2
    - name: PHP-CS-Fixer
      uses: docker://oskarstark/php-cs-fixer-ga

to use a custom config for example, --diff and --dry-run option:

on: [push, pull_request]
name: Main
jobs:
  php-cs-fixer:
    name: PHP-CS-Fixer
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2
    - name: PHP-CS-Fixer
      uses: docker://oskarstark/php-cs-fixer-ga
+      with:
+        args: --config=.project.php_cs --diff --dry-run

You can copy/paste the .github folder (under examples/) to your project and thats all!

A Docker-Image is built automatically and located here: https://hub.docker.com/r/oskarstark/php-cs-fixer-ga

You can run it in any given directory like this:

docker run --rm -it -w=/app -v ${PWD}:/app oskarstark/php-cs-fixer-ga:latest

You can find a working and not working PR here: https://github.com/OskarStark/test-php-cs-fixer-ga/pulls