Cache Docker dependency downloads in embedded images (#7535)

# Description of Changes

Adds Docker BuildKit syntax and shared Gradle/npm cache mounts to the
fat and ultra-lite embedded Dockerfiles. This reduces repeated
dependency downloads during rebuilds and speeds up image builds without
changing runtime behavior.

---

## 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/docker-compose-tests.yml

@@ -66,7 +66,7 @@ jobs:
 
       - name: Install Docker Compose
         run: |
-          sudo curl -SL "https://github.com/docker/compose/releases/download/v2.39.4/docker-compose-$(uname -s)-$(uname -m)" -o /usr/local/bin/docker-compose
+          sudo curl -SL "https://github.com/docker/compose/releases/download/v5.4.0/docker-compose-$(uname -s)-$(uname -m)" -o /usr/local/bin/docker-compose
           sudo chmod +x /usr/local/bin/docker-compose
 
       - name: Install uv

docker/embedded/Dockerfile.fat

@@ -1,3 +1,5 @@
+# syntax=docker/dockerfile:1
+
 # Stirling-PDF - Fat version (embedded frontend)
 # Extra fonts for air-gapped environments
 # Uses pre-built base image for fast builds
@@ -38,7 +40,8 @@ COPY app/common/build.gradle                app/common/
 COPY app/proprietary/build.gradle           app/proprietary/
 
 # Use system gradle instead of gradlew to avoid SSL issues downloading gradle distribution on emulated arm64
-RUN gradle dependencies --no-daemon || true
+RUN --mount=type=cache,id=stirling-pdf-gradle-cache,target=/home/gradle/.gradle,sharing=locked \
+    gradle dependencies --no-daemon || true
 
 COPY . .
 
@@ -46,7 +49,9 @@ COPY . .
 ARG BUILD_PORTAL=false
 # Bundle only the JPDFium native for this image's target arch.
 ARG TARGETARCH
-RUN JPDFIUM_PLATFORM="$([ "$TARGETARCH" = arm64 ] && echo linux-arm64 || echo linux-x64)" && \
+RUN --mount=type=cache,id=stirling-pdf-npm-cache,target=/root/.npm,sharing=locked \
+    --mount=type=cache,id=stirling-pdf-gradle-cache,target=/home/gradle/.gradle,sharing=locked \
+    JPDFIUM_PLATFORM="$([ "$TARGETARCH" = arm64 ] && echo linux-arm64 || echo linux-x64)" && \
     DISABLE_ADDITIONAL_FEATURES=false \
     gradle clean build \
       -PbuildWithFrontend=true \

docker/embedded/Dockerfile.ultra-lite

@@ -1,3 +1,5 @@
+# syntax=docker/dockerfile:1
+
 # Stirling-PDF Dockerfile - Ultra-lite version with embedded frontend
 # Single JAR contains both frontend and backend with minimal dependencies
 
@@ -36,7 +38,8 @@ ENV JDK_JAVA_OPTIONS="--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNN
   --add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED \
   --add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED"
 
-RUN ./gradlew dependencies --no-daemon || true
+RUN --mount=type=cache,id=stirling-pdf-gradle-cache,target=/home/gradle/.gradle,sharing=locked \
+    ./gradlew dependencies --no-daemon || true
 
 # Copy entire project
 COPY . .
@@ -46,7 +49,9 @@ COPY . .
 ARG BUILD_PORTAL=false
 # Bundle only the JPDFium native for this image's target arch.
 ARG TARGETARCH
-RUN JPDFIUM_PLATFORM="$([ "$TARGETARCH" = arm64 ] && echo linux-arm64 || echo linux-x64)" && \
+RUN --mount=type=cache,id=stirling-pdf-npm-cache,target=/root/.npm,sharing=locked \
+    --mount=type=cache,id=stirling-pdf-gradle-cache,target=/home/gradle/.gradle,sharing=locked \
+    JPDFIUM_PLATFORM="$([ "$TARGETARCH" = arm64 ] && echo linux-arm64 || echo linux-x64)" && \
     DISABLE_ADDITIONAL_FEATURES=true \
     ./gradlew clean build \
       -PbuildWithFrontend=true \