Fix PR docker CI when the base image changes (#6809)

# Description of Changes

- Fix PR CI for base-image changes: the embedded build's buildx
container builder could not resolve the locally-built
`stirling-pdf-base:pr-test` and tried to pull it from a registry,
failing the build
- `test-build-docker.yml`: when the base changed, build the embedded
image with the docker driver (`docker build`) so the locally-built base
resolves from the daemon image store
- `docker-compose-tests.yml`: when the base changed, skip the buildx
container builder + gha cache so `test.sh`'s local base build resolves
via the default docker driver

---

## Checklist

### General

- [x] 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/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)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs (if functionality has heavily
changed)
- [ ] I have read the section Add New Translation Tags (for new
translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally

---------

Co-authored-by: James Brunton <jbrunton96@gmail.com>

.github/workflows/docker-compose-tests.yml

@@ -64,11 +64,19 @@ jobs:
           gradle-version: 9.6.0
           cache-disabled: true
 
+      # When the PR changes the base image, test.sh builds it locally
+      # (stirling-pdf-base:local) into the daemon image store. A buildx
+      # container builder can't see that store, so skip it here and let
+      # `docker buildx build` fall back to the default docker driver, which
+      # resolves the local base. The gha cache backend is also skipped (its
+      # runtime token isn't exposed) since the docker driver can't use it.
       - name: Set up Docker Buildx
+        if: inputs.docker-base-changed != 'true'
         uses: docker/setup-buildx-action@4d04d5d9486b7bd6fa91e7baf45bbb4f8b9deedd # v4.0.0
 
       # Expose ACTIONS_RUNTIME_TOKEN / ACTIONS_RESULTS_URL for docker buildx type=gha cache backend.
       - name: Expose GitHub runtime for Buildx cache
+        if: inputs.docker-base-changed != 'true'
         uses: crazy-max/ghaction-github-runtime@04d248b84655b509d8c44dc1d6f990c879747487 # v4.0.0
 
       - name: Install Docker Compose