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/Dockerfile

@@ -0,0 +1,116 @@
+# ─────────────────────────────────────────────────────────────────────────────
+# photo-converter — Multi-Stage Build Image
+#
+# Stages:
+#   base            — Gemeinsame Tools (cmake, ninja, gcc)
+#   linux-builder   — Linux-Build mit OpenCV, LibRaw, Qt6
+#   windows-builder — Windows Cross-Compilation via MXE + MinGW-w64
+#
+# Verwendung:
+#   docker build --target linux-builder  -t photo-converter:linux   .
+#   docker build --target windows-builder -t photo-converter:windows .
+# ─────────────────────────────────────────────────────────────────────────────
+
+# Ubuntu 22.04 LTS: MXE-Pakete sind für focal/jammy verfügbar
+ARG UBUNTU_VERSION=22.04
+FROM ubuntu:${UBUNTU_VERSION} AS base
+
+ENV DEBIAN_FRONTEND=noninteractive \
+    TZ=UTC
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    # Build-Tools
+    cmake \
+    ninja-build \
+    build-essential \
+    pkg-config \
+    git \
+    # Hilfswerkzeuge
+    wget \
+    curl \
+    unzip \
+    zip \
+    ca-certificates \
+    gnupg \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /project
+
+# ─────────────────────────────────────────────────────────────────────────────
+# Stage: linux-builder
+# Linux-Build mit OpenCV, LibRaw, Qt6
+# ─────────────────────────────────────────────────────────────────────────────
+FROM base AS linux-builder
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    # OpenCV
+    libopencv-dev \
+    # LibRaw
+    libraw-dev \
+    # Qt6
+    qt6-base-dev \
+    libqt6widgets6 \
+    libxkbcommon-dev \
+    # GTest für Tests
+    libgtest-dev \
+    # OpenGL (Qt-Abhängigkeit)
+    libgl1-mesa-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY . /project
+
+# Standard: Release ohne GUI (GUI benötigt X11/Display)
+RUN cmake -B build -G Ninja \
+        -DCMAKE_BUILD_TYPE=Release \
+        -DBUILD_GUI=OFF \
+        -DBUILD_TESTS=ON \
+    && cmake --build build --parallel "$(nproc)" \
+    && ctest --test-dir build --output-on-failure
+
+ENTRYPOINT ["./build/photo-converter"]
+CMD ["--batch", "--config", "config.ini"]
+
+# ─────────────────────────────────────────────────────────────────────────────
+# Stage: windows-builder
+# Windows Cross-Compilation via MXE (MinGW-w64 + statisch gelinkte Deps)
+# ─────────────────────────────────────────────────────────────────────────────
+FROM base AS windows-builder
+
+# MXE Repository einrichten
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    apt-transport-https \
+    lsb-release \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN curl -fsSL "https://pkg.mxe.cc/repos/apt/client-conf/mxeapt.gpg" \
+        -o /etc/apt/trusted.gpg.d/mxeapt.gpg \
+    && echo "deb [arch=amd64] https://pkg.mxe.cc/repos/apt focal main" \
+        > /etc/apt/sources.list.d/mxeapt.list
+
+# MXE-Pakete: OpenCV + LibRaw + Qt6 (statisch, x86_64)
+# Hinweis: Initial-Download ~3 GB, dauert je nach Bandbreite 10-30 Min.
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    mxe-x86-64-w64-mingw32.static-cmake \
+    mxe-x86-64-w64-mingw32.static-opencv4 \
+    mxe-x86-64-w64-mingw32.static-libraw \
+    mxe-x86-64-w64-mingw32.static-qtbase \
+    mxe-x86-64-w64-mingw32.static-cc \
+    && rm -rf /var/lib/apt/lists/*
+
+ENV MXE_PREFIX=/usr/lib/mxe/usr/x86_64-w64-mingw32.static \
+    PATH="/usr/lib/mxe/usr/bin:${PATH}"
+
+COPY . /project
+
+RUN cmake -B build-windows -G Ninja \
+        -DCMAKE_TOOLCHAIN_FILE=cmake/toolchain-mingw64.cmake \
+        -DCMAKE_BUILD_TYPE=Release \
+        -DCMAKE_PREFIX_PATH="${MXE_PREFIX}" \
+        -DCMAKE_INSTALL_PREFIX=/project/dist-windows \
+        -DBUILD_GUI=ON \
+        -DBUILD_TESTS=OFF \
+    && cmake --build build-windows --parallel "$(nproc)" \
+    && cmake --install build-windows
+
+# Ergebnis: /project/dist-windows/bin/photo-converter.exe
+ENTRYPOINT ["/bin/bash"]