Building MITK on Ubuntu 20.04 using the system packages of Qt instead of the installer works fine.
However, make package fails since the directory layout of the deb packages differs from the standard Qt install:
CMake Error at /opt/MITK-superbuild/MITK-build/cmake_install.cmake:5531 (file): file INSTALL cannot find "/usr/lib/x86_64-linux-gnu/qt5/bin/../resources": No such file or directory. CMake Error at /opt/MITK-superbuild/MITK-build/cmake_install.cmake:5535 (file): file INSTALL cannot find "/usr/lib/x86_64-linux-gnu/qt5/bin/../translations/qtwebengine_locales": No such file or directory.
To reproduce install the necessary packages on Ubuntu:
qtbase5-dev qtscript5-dev libqt5svg5-dev libqt5opengl5-dev libqt5xmlpatterns5-dev qtwebengine5-dev qttools5-dev libqt5x11extras5-dev qtxmlpatterns5-dev-tools libqt5webengine-data
This requires a hotfix since it currently blocks MITK container builds for Kaapana (internal)