- 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).