…simplify PR test-build deps (#4879)

# Description of Changes

This pull request updates the Docker setup for all image variants
(`Dockerfile`, `Dockerfile.fat`, and `Dockerfile.ultra-lite`) to improve
shell compatibility and ensure consistent behavior across environments.
The most important changes are grouped below:

Shell compatibility improvements:

* Added installation of `bash` and created a symlink from `/bin/bash` to
`/bin/sh` to ensure scripts expecting `bash` work correctly in all
Docker images. (`Dockerfile`:
[[1]](diffhunk://#diff-dd2c0eb6ea5cfc6c4bd4eac30934e2d5746747af48fef6da689e85b752f39557L42-R44)
`Dockerfile.fat`:
[[2]](diffhunk://#diff-571631582b988e88c52c86960cc083b0b8fa63cf88f056f26e9e684195221c27L56-R58)
`Dockerfile.ultra-lite`:
[[3]](diffhunk://#diff-ebe2e5849a198a8b5be70f42dcd1ddc518c7584d525f6492d221db9f7a45a6f5L27-R29)

Default shell consistency:

* Added a symlink from `/bin/busybox` to `/bin/sh` after setting user
permissions to ensure the default shell remains available and consistent
for system utilities. (`Dockerfile`:
[[1]](diffhunk://#diff-dd2c0eb6ea5cfc6c4bd4eac30934e2d5746747af48fef6da689e85b752f39557L101-R104)
`Dockerfile.fat`:
[[2]](diffhunk://#diff-571631582b988e88c52c86960cc083b0b8fa63cf88f056f26e9e684195221c27L114-R117)
`Dockerfile.ultra-lite`:
[[3]](diffhunk://#diff-ebe2e5849a198a8b5be70f42dcd1ddc518c7584d525f6492d221db9f7a45a6f5L47-R50)

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.