The correct option is Docker expects a file named Dockerfile with an uppercase D and it fails when the file is named dockerfile.

This is the most likely cause because Docker looks for a default build file named Dockerfile and GitHub hosted runners are case sensitive. When the repository contains a file named dockerfile with a lowercase d the build step does not find Dockerfile and the container action fails during image build. You could use a different filename only if you explicitly direct Docker to it which the standard container action setup does not do.

The workflow must use Cloud Build to build the container image is incorrect because GitHub Actions builds container images with Docker on the runner and it does not require an external build service.

The action.yml file is ignored by container actions which causes the run to fail is incorrect because container actions require the metadata file and GitHub reads it to determine inputs and how to execute the action.

The script.sh file must be referenced in the README for it to be executed is incorrect because documentation files do not control execution. The script is run by the container entry point or by commands defined in the build and metadata.

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