try ON instead of TRUE perhaps then CI works?

.gitlab-ci.yml

 ---
 variables:
   # dune-python will use DUNE_OPTS when building stuff
-  DUNECI_CMAKE_FLAGS: "-DDUNE_ENABLE_PYTHONBINDINGS=TRUE -DDUNE_MAX_TEST_CORES=4 -DBUILD_SHARED_LIBS=TRUE -DDUNE_PYTHON_INSTALL_LOCATION=none -DCMAKE_POSITION_INDEPENDENT_CODE=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_LATEX=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Alberta=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Vc=TRUE -DCMAKE_DISABLE_DOCUMENTATION=TRUE"
+  DUNECI_CMAKE_FLAGS: "-DDUNE_ENABLE_PYTHONBINDINGS=ON -DDUNE_MAX_TEST_CORES=4 -DBUILD_SHARED_LIBS=TRUE -DDUNE_PYTHON_INSTALL_LOCATION=none -DCMAKE_POSITION_INDEPENDENT_CODE=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_LATEX=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Alberta=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Vc=TRUE -DCMAKE_DISABLE_DOCUMENTATION=TRUE"
   DUNECI_TOOLCHAIN:  gcc-7-17
   DUNE_CONTROL_PATH: /duneci/modules:$CI_PROJECT_DIR
   DUNE_PY_DIR:       /duneci/modules/dune-py

CMakeLists.txt

@@ -5,7 +5,7 @@ project("dune-fem-dg" C CXX)
 option(DUNE_GRID_GRIDTYPE_SELECTOR "Grid selector definition added to config.h" ON)
 
 # general stuff
-cmake_minimum_required(VERSION 2.8.12)
+cmake_minimum_required(VERSION 3.1)
 
 #find dune-common and set the module path
 find_package(dune-common)