The metadata file that defines the main entry point for a custom GitHub Action is action.yml (or action.yaml).
This file, located in the root directory of your action's repository, uses YAML syntax and includes a runs key that specifies how the action is executed and which script is the main entry point.
Key fields in the action.yml file
name: The name of your action displayed in the GitHub Actions UI.
description: A short description of the action.
runs: This required key specifies the execution environment and the main entry point.
For a JavaScript action, the runs field includes using (e.g., node20) and main (e.g., index.js) to define the runtime and the primary script file.
For a Docker container action, the runs field includes using: 'docker' and image (e.g., 'Dockerfile') to point to the Docker build file or image.
inputs: Optional parameters that allow users to pass data to your action.
outputs: Optional output parameters that the action can produce for use by subsequent steps in a workflow.
Reference:
https://docs.github.com/en/actions/reference/workflows-and-actions/metadata-syntax