Add Docker example

[joseluisq]

Description

This PR just adds a Docker example.

Motivation and Context

Missing Docker image cache example.

How Has This Been Tested?

In several of my projects like this https://github.com/joseluisq/alpine-php-fpm/blob/master/.github/workflows/devel-8.2.yml#L45-L76

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation (add or update README or docs)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.