ci: Add Docker build workflow to test all image variants

Tests all three Dockerfile build types in parallel:
- minimal: Basic CCExtractor without OCR
- ocr: CCExtractor with Tesseract OCR support
- hardsubx: CCExtractor with burned-in subtitle extraction

Each job builds from local source and verifies the image works
by running --version. Uses GitHub Actions cache for faster rebuilds.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/build_docker.yml

@@ -0,0 +1,92 @@
+name: Build CCExtractor Docker Images
+
+on:
+  workflow_dispatch:
+  push:
+    paths:
+    - '.github/workflows/build_docker.yml'
+    - 'docker/**'
+    - '**.c'
+    - '**.h'
+    - 'src/rust/**'
+  pull_request:
+    types: [opened, synchronize, reopened]
+    paths:
+    - '.github/workflows/build_docker.yml'
+    - 'docker/**'
+    - '**.c'
+    - '**.h'
+    - 'src/rust/**'
+
+jobs:
+  build_minimal:
+    name: Docker build (minimal)
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v6
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
+    - name: Build minimal image
+      uses: docker/build-push-action@v6
+      with:
+        context: .
+        file: docker/Dockerfile
+        build-args: |
+          BUILD_TYPE=minimal
+          USE_LOCAL_SOURCE=1
+        tags: ccextractor:minimal
+        load: true
+        cache-from: type=gha,scope=docker-minimal
+        cache-to: type=gha,mode=max,scope=docker-minimal
+    - name: Test minimal image
+      run: |
+        docker run --rm ccextractor:minimal --version
+        echo "Minimal build successful"
+
+  build_ocr:
+    name: Docker build (ocr)
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v6
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
+    - name: Build OCR image
+      uses: docker/build-push-action@v6
+      with:
+        context: .
+        file: docker/Dockerfile
+        build-args: |
+          BUILD_TYPE=ocr
+          USE_LOCAL_SOURCE=1
+        tags: ccextractor:ocr
+        load: true
+        cache-from: type=gha,scope=docker-ocr
+        cache-to: type=gha,mode=max,scope=docker-ocr
+    - name: Test OCR image
+      run: |
+        docker run --rm ccextractor:ocr --version
+        echo "OCR build successful"
+
+  build_hardsubx:
+    name: Docker build (hardsubx)
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v6
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
+    - name: Build HardSubX image
+      uses: docker/build-push-action@v6
+      with:
+        context: .
+        file: docker/Dockerfile
+        build-args: |
+          BUILD_TYPE=hardsubx
+          USE_LOCAL_SOURCE=1
+        tags: ccextractor:hardsubx
+        load: true
+        cache-from: type=gha,scope=docker-hardsubx
+        cache-to: type=gha,mode=max,scope=docker-hardsubx
+    - name: Test HardSubX image
+      run: |
+        docker run --rm ccextractor:hardsubx --version
+        echo "HardSubX build successful"