ci: share Docker base image across test-build matrix (#7584)

# Description of Changes

This PR decouples the Docker base-image preparation from the embedded
Docker image matrix builds in `.github/workflows/test-build-docker.yml`.

- Added a dedicated `prepare-base-image` job that runs once when a pull
request changes the Docker base image.
- Builds `stirling-pdf-base:pr-test` once for `linux/amd64` instead of
rebuilding the same image independently in every matrix job.
- Exports the prepared image with `docker save`, compresses it, and
uploads it as a short-lived GitHub Actions artifact.
- Added a dependency from `test-build-docker-images` to
`prepare-base-image`, while still allowing the matrix job to run when
base-image preparation is skipped.
- Each matrix entry downloads and loads the prepared Docker image when
`docker-base-changed` is enabled.
- Removed the previous per-matrix `Build base image locally` step.
- Keeps the prepared image available to the embedded Docker builds
through the local Docker daemon.

The change was made to eliminate redundant base-image builds across the
Docker test matrix. Previously, pull requests modifying `docker/base`
caused each matrix entry to build the identical base image independently
and in parallel. Preparing the image once reduces duplicated CI work,
improves consistency between matrix entries, and should reduce CI
resource usage and execution time for Docker-related pull requests.

---

## 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/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 run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

.github/workflows/test-build-docker.yml

@@ -22,21 +22,42 @@ permissions:
   contents: read
 
 jobs:
-  # TODO: extract a pre-matrix `prepare` job that runs once and produces
-  # shared artifacts for the three matrix entries below to consume:
-  #   1. `task backend:build` — currently runs 3× in parallel with
-  #      identical env (DISABLE_ADDITIONAL_FEATURES=true,
-  #      STIRLING_PDF_DESKTOP_UI=false). Build once, upload the JAR as an
-  #      artifact, matrix entries download.
-  #   2. The base-image `docker build` (gated on docker-base-changed) —
-  #      currently runs 3× in parallel against the same Dockerfile and
-  #      context. Build once, `docker save` to an artifact, matrix entries
-  #      `docker load` before the embedded build.
-  # Saves ~2 full backend builds + 2 base-image builds per PR that touches
-  # docker. May also be reusable from backend-build.yml's jdk-25 +
-  # spring-security=true matrix entry if `task backend:build` and
-  # `task backend:build:ci` produce equivalent JARs (verify before wiring).
+  # A changed base image is shared by all three embedded-image builds. Build
+  # it once and transfer it as an artifact; the matrix jobs use the local
+  # Docker driver so the loaded image is visible to the build.
+  prepare-base-image:
+    if: github.event_name == 'pull_request' && inputs.docker-base-changed == 'true'
+    environment:
+      name: ci-unsigned
+      deployment: false
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@b09bb98e06d4d774595224525879c09bc6e98c40 # v2.20.1
+        with:
+          egress-policy: audit
+
+      - name: Checkout Repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Build base image locally
+        run: docker build --platform linux/amd64 -t stirling-pdf-base:pr-test -f docker/base/Dockerfile docker/base
+
+      - name: Export base image
+        run: docker save stirling-pdf-base:pr-test | gzip -1 > stirling-pdf-base-pr-test.tar.gz
+
+      - name: Upload base image
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: docker-base-pr-test
+          path: stirling-pdf-base-pr-test.tar.gz
+          retention-days: 1
+          if-no-files-found: error
+
   test-build-docker-images:
+    if: always() && (needs.prepare-base-image.result == 'success' || needs.prepare-base-image.result == 'skipped')
+    needs: [prepare-base-image]
     environment:
       name: ci-unsigned
       deployment: false
@@ -81,6 +102,16 @@ jobs:
           docker system prune -af || true
           echo "Disk space after cleanup:" && df -h
 
+      - name: Download prepared base image
+        if: github.event_name == 'pull_request' && inputs.docker-base-changed == 'true'
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
+        with:
+          name: docker-base-pr-test
+
+      - name: Load prepared base image
+        if: github.event_name == 'pull_request' && inputs.docker-base-changed == 'true'
+        run: gzip -dc stirling-pdf-base-pr-test.tar.gz | docker load
+
       - name: Restore cache Gradle User Home
         uses: actions/cache/restore@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
         with:
@@ -113,11 +144,6 @@ jobs:
         id: buildx
         uses: docker/setup-buildx-action@bb05f3f5519dd87d3ba754cc423b652a5edd6d2c # v4.2.0
 
-      - name: Build base image locally (PR base change only)
-        if: github.event_name == 'pull_request' && inputs.docker-base-changed == 'true'
-        run: |
-          docker build -t stirling-pdf-base:pr-test -f docker/base/Dockerfile docker/base
-
       - name: Set base image and platform for this build
         id: build-params
         # Pass workflow inputs through env vars rather than expanding `${{ }}`