feat: containerize build environment with Docker

- docker/Dockerfile: multi-stage image (linux-builder + windows-builder)
  - linux-builder: Ubuntu 22.04 + OpenCV/LibRaw/Qt6 + GTest
  - windows-builder: MXE cross-compilation (x86_64-w64-mingw32.static)
    with OpenCV4, LibRaw, Qt6 for Windows .exe output
- docker-compose.yml: services for linux, windows-build, shell
- scripts/docker-build.sh: convenience wrapper
  - linux|windows|all targets
  - --no-cache, --run flags

Usage:
  ./scripts/docker-build.sh linux --run
  ./scripts/docker-build.sh windows

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,52 @@
+name: photo-converter
+
+services:
+
+  # ── Linux-Build (CLI) ────────────────────────────────────────────────────
+  linux:
+    build:
+      context: .
+      dockerfile: docker/Dockerfile
+      target: linux-builder
+    image: photo-converter:linux
+    volumes:
+      # Input-Bilder aus lokalem import/ Ordner einbinden
+      - ./import:/project/import:ro
+      # Konvertierte Bilder in lokalen output/ Ordner schreiben
+      - ./output:/project/output
+      # Konfiguration überschreibbar
+      - ./config.ini:/project/config.ini:ro
+    command: ["--batch", "--config", "config.ini"]
+
+  # ── Windows Cross-Compilation ────────────────────────────────────────────
+  windows-build:
+    build:
+      context: .
+      dockerfile: docker/Dockerfile
+      target: windows-builder
+    image: photo-converter:windows-builder
+    volumes:
+      # Windows-Build-Ergebnis in lokales dist-windows/ schreiben
+      - ./dist-windows:/project/dist-windows
+    # Nur bauen, kein Entrypoint
+    entrypoint: ["/bin/bash", "-c"]
+    command:
+      - |
+        echo "Windows-Build abgeschlossen."
+        echo "Ergebnis in: /project/dist-windows"
+        ls -lh /project/dist-windows/bin/ 2>/dev/null || echo "(kein bin/ Verzeichnis)"
+
+  # ── Interaktive Shell (Debugging) ────────────────────────────────────────
+  shell:
+    build:
+      context: .
+      dockerfile: docker/Dockerfile
+      target: linux-builder
+    image: photo-converter:linux
+    volumes:
+      - ./import:/project/import:ro
+      - ./output:/project/output
+      - ./config.ini:/project/config.ini:ro
+    entrypoint: ["/bin/bash"]
+    stdin_open: true
+    tty: true