Update docker base image

.docker/Dockerfile

-FROM debian:buster-backports
+FROM debian:bullseye
 
 RUN apt-get update
-RUN apt-get install -y cmake/buster-backports
+RUN apt-get install -y cmake
 RUN apt-get install -y lcov
 RUN apt-get install -y doxygen
 RUN apt-get install -y clang-format-11 clang-tidy-11

test/CMakeLists.txt

@@ -112,5 +112,8 @@ if (LCOV_PATH)
 else ()
     if (NOT SKIP_CODE_COVERAGE)
         message(WARNING "Could not generate code coverage report. Please install lcov.")
+        add_custom_target(unit_test_coverage ctest -L caosdb-cpplib-unit-tests
+            DEPENDS caosdb ccaosdb ${test_cases}
+        )
     endif()
 endif ()