documentation for building / using the qt5 image locally

[thewtex]

Here are notes from commands that we used (needs testing):

sudo mkdir /work
sudo chown -R $USER.$USER /work
cd /work
git clone https://github.com/Slicer/Slicer.git
docker run --rm slicer/buildenv-qt5-centos7 > slicer-buildenv-qt5-centos7
chmod +x ./slicer-buildenv-qt5-centos7
./slicer-buildenv-qt5-centos7 cmake -BSlicer-build -HSlicer -GNinja -DCMAKE_BUILD_TYPE=Release -DSlicer_USE_PYTHONQT_WITH_TCL:BOOL=OFF -DSlicer_BUILD_CLI:BOOL=OFF -DSlicer_USE_SimpleITK:BOOL=OFF -DBUILD_TESTING:BOOL=OFF -DQt5_DIR:PATH=\${Qt5_DIR}
./slicer-buildenv-qt5-centos7 cmake --build Slicer-build
# mv /opt/qt from the image onto the host