Remove need to manually write workflow definition for `docker build` test

* Currently, pull-request workflows in https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/tree/main/.github/workflows that test `docker build`ing of Dockerfiles are written manually for each new Dockerfile added into this repo.
* Instead, we may be able to write some code _once_ and have all Dockerfiles (even those added in the future) tested in pull-requests.
* This removes to need for comments like this: https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/pull/1585#discussion_r1909317913