Update Docker Setup for Cachet: Improved Architecture, Workflow, and Documentation and support 3.x and higher

[steffjenl]

This pull request updates the Docker setup for Cachet with the following improvements:

• Refactored architecture to separate the application, scheduler, and worker into individual containers for better isolation, scalability, and maintainability.
• Enhanced documentation in README.md with architecture rationale, production best practices, and maintenance instructions.
• Updated GitHub Actions workflow (.github/workflows/docker-publish.yml) for multi-platform image builds, improved version handling, and secure registry logins.
• Added security recommendations and reverse proxy setup examples.
• Improved backup, restore, and troubleshooting sections.

These changes make the deployment more robust, secure, and easier to manage in production environments.
Please review and merge into the main project.

[steffjenl]

@jbrooksuk cleanup the PR.