Support using a Container Action input in field runs: { image: ... }

[umarcor]

Container Actions are defined by an action.yml configuration file with fields runs: { using: 'docker', image: ... }: https://docs.github.com/en/actions/creating-actions/creating-a-docker-container-action. Unfortunately, inputs cannot be used in field image, which prevents users from passing the image to be used as an argument.

# action.yml
inputs:
  image:
    description: 'Which container image to use'
    required: true
    default: 'debian:bullseye-slim'
runs:
  using: 'docker'
  image: '${{ inputs.image }}'

In order to provide such functionality, a Composite Action can be used as a workaround (see, for instance, https://github.com/VUnit/vunit_action/blob/master/action.yml). However, that requires Action developers to duplicate the built-in functionality with regard to settings envvars, bindings, workdir, etc. inside the container.

This is partially related to #9049, because supporting inputs in the uses fields of Composite Actions would allow to use Container Steps.