Install qt6, to which we will soon transition everything.

Dockerfile

@@ -65,6 +65,9 @@ RUN apt-get update && apt-get install -y libhepmc3-dev
 ### Install libegl-dev to detect graphics drivers in gmex
 RUN apt-get update && apt-get install -y libegl-dev
 
+### Install qt6 (co-exist with qt5 during transition period)
+RUN apt-get update && apt get install -y qt6-base-dev libqt6core5compat6-dev libqt6opengl6-dev
+
 ### Install xercesC
 RUN mkdir -p build_xercesC  && \
     cd build_xercesC && \