feat: Allow overriding docker hub hostname

What

• Allow override of docker hub hostname to use features like dependency proxy or another proxied registry
• Example of how to use:

  - project: "gitlab-com/gl-infra/common-ci-tasks"
    ref: v2.8.0  # renovate:managed
    file: templates/standard.yml
    inputs:
      docker_hub_host: ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}

Why

• Avoiding rate limit errors while dogfooding hosted runners for dedicated