Docker Hubの自動ビルドは2021/06/18以降、無償でできなくなる
Unfortunately, Docker’s Autobuild service has been targeted by the same bad actors, so today we are disappointed to announce that we will be discontinuing Autobuilds on the free tier starting from June 18, 2021.
Docker Hubの自動ビルドについては以下のドキュメントが詳しい（公式）