You maintain a Docker container action that builds from a `Dockerfile` in the repository root. It works on your local machine and on self-hosted ARM64 runners, but every run on the standard GitHub-hosted `ubuntu-latest` runner fails immediately during the container build step with an 'exec format error' when the entrypoint script is invoked. The action.yml declares `runs.using: docker` and `runs.image: Dockerfile`. What is the most likely cause and appropriate fix?