Split the prebuilt-image recipe out of new_docker_build.sh into a Dockerfile

- Dockerfile: the prebuilt build image recipe (Ubuntu 26.04 + apt deps +
  aqtinstall Qt6 + KDReports), parameterized via ARG APT_PACKAGES /
  QT_VERSION / QT_ARCH / QT_BASE_DIR.
- new_docker_build.sh: build_prebuilt_image() now runs
  docker build -f Dockerfile with the package list and Qt settings passed
  as build args, using an empty build context (the Dockerfile has no
  COPY/ADD).

qt6-after/qt6-migrated-legatus/Dockerfile

@@ -0,0 +1,52 @@
+# Prebuilt build image for the release validation-scope branch
+# (Ubuntu 26.04 + Qt6 + all dependencies for the legatus release build).
+#
+# Built by new_docker_build.sh --build-prebuilt-image, which passes the
+# package list and Qt settings as build args (see build_prebuilt_image()).
+
+FROM ubuntu:26.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+ARG APT_PACKAGES
+ARG QT_VERSION=6.10.2
+ARG QT_ARCH=linux_gcc_64
+ARG QT_BASE_DIR=/opt/Qt
+
+RUN apt-get update -qq && \
+    apt-get install -y -qq \
+      ${APT_PACKAGES} \
+      libqt6svg6-dev \
+      libgtest-dev \
+      libgmock-dev \
+      python3-pip && \
+    rm -rf /var/lib/apt/lists/*
+
+# Install Qt6 packages via aqtinstall
+# Note: QtWebKit is deprecated in Qt6, use QtWebEngine instead
+# KDReports is installed via aqtinstall or built from source if needed
+# Note: qtsvg is NOT available via aqtinstall - it's provided by libqt6svg6-dev apt package
+RUN pip3 install --break-system-packages --quiet aqtinstall && \
+    aqt install-qt linux desktop ${QT_VERSION} ${QT_ARCH} \
+      -m qtmultimedia qtserialport qtimageformats qtwebsockets qtwebengine \
+         qtpdf qtwebview qt5compat qtwebchannel qtpositioning qtlocation qtsensors \
+      --outputdir ${QT_BASE_DIR} && \
+    rm -rf /root/.cache/Aqt
+
+# Install KDReports from source (apt package not available in Ubuntu 22.04/26.04)
+# Try apt install first (will fail), then build from KDAB GitHub mirror
+RUN apt-get install -y -qq libkdreports-dev 2>/dev/null || \
+    (git clone --depth 1 https://github.com/KDAB/KDReports.git /tmp/KDReports && \
+     cd /tmp/KDReports && \
+     mkdir build && cd build && \
+     cmake -DCMAKE_INSTALL_PREFIX=/usr/local -DCMAKE_PREFIX_PATH=${QT_BASE_DIR} .. && \
+     make -j$(nproc) && \
+     make install && \
+     cd / && rm -rf /tmp/KDReports)
+
+# Legacy sources include <KDReports/...> but the Qt6 package installs headers flat
+# under /usr/local/include/KDReports-qt6 - alias the directory so the prefixed
+# includes resolve through the default /usr/local/include search path.
+RUN ln -sfn KDReports-qt6 /usr/local/include/KDReports
+
+CMD ["/bin/bash"]