Enabled update checking via Dependabot for
* GitHub Actions for updates or in case you are going to use that (just my default)
* PHP aka Composer
We successfully use it [for our Docker container](https://github.com/PrivateBin/docker-nginx-fpm-alpine/blob/master/.github/dependabot.yml) and IMHO that has worked quite well, so IMHO, we can also use it here now.
