Remove need to manually write workflow definition for docker build test

[NimJay]

• Currently, pull-request workflows in https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/tree/main/.github/workflows that test docker building 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: Update the fine-tuning dataset and codebase, remove terraform related resources #1585 (comment)