Strengthen CI with shell linting, multi-arch smoke tests, and gated image publishing

[Copilot]

• Revisar el workflow actual, el smoke test y la integración de ZAP
• Ejecutar lint y validaciones existentes antes de cambiar nada
• Sustituir el job de ZAP por una comprobación web/PHP más simple y mantenible
• Mantener el gate de CI para que publish dependa de esa validación
• Revalidar YAML, scripts y diff final
• Ejecutar code review y CodeQL, corregir hallazgos y cerrar

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve Automatic Tests for CI on alpine-php-webserver Using GitHub Actions</issue_title>
<issue_description>The current setup for Continuous Integration (CI) in the alpine-php-webserver repository could be enhanced to provide more robust and comprehensive automated testing for the Docker image. Improving the testing process will help ensure higher reliability and prevent regressions when introducing new features or updates.

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Proposed Improvements

1. Add Multi-Platform Build and Tests:

   • Ensure the image is tested on multiple architectures (amd64, arm64, etc.) using Docker Buildx.
   • Validate compatibility and functionality across platforms.

2. Lint Dockerfile and Shell Scripts:

   • Use tools like [hadolint](https://github.com/hadolint/hadolint) for Dockerfile linting.
   • Include shellcheck for any included scripts (e.g., entrypoints or helper scripts).

3. Run Containerized Tests:

   • Test the Docker image by running it and executing basic PHP scripts.
   • Verify Nginx and PHP-FPM functionality by simulating web requests using tools like curl or PHPUnit tests for web servers.

4. Check for Vulnerabilities:

   • Integrate vulnerability scanning (e.g., with [trivy](https://github.com/aquasecurity/trivy)) to identify any security issues in the built image.

5. Cache Optimization:

   • Implement caching strategies for Docker layers and Composer dependencies during the CI process to speed up builds.

6. Automate Publishing on Successful Tests:

   • Automatically publish the Docker image to the registry only if all tests pass and on specific triggers (e.g., tags or branches like main).

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Acceptance Criteria

• CI workflow runs tests for multi-platform builds and passes without errors.
• Dockerfile and scripts are linted during the CI workflow.
• Tests verify PHP and Nginx configurations work as expected.
• Vulnerability scan integrates into the CI pipeline with clear reporting.
• Workflow outputs are clear and informative for debugging.

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Additional Context

Implementing these improvements will align the repository with modern CI/CD practices and increase confidence in the image's stability and security. It also provides contributors with immediate feedback on their changes.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve Automatic Tests for CI on alpine-php-webserver Using GitHub Actions #35

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