Merge branch 'feature/ubuntu-18.04-cmake-update' into 'master'

ref:docker/ci\>

### Summary

This MR adds a recent cmake version to the ubuntu 18.04 distribution.

### Details

Ubuntu 18.04 ships with cmake 3.10. For a development of a new build-system we
might need at least cmake 3.13. This MR updates the cmake version by adding a
kitware apt repository. This way we get cmake 3.17

See merge request [docker/ci!61]

  [docker/ci!61]: gitlab.dune-project.org/docker/ci/merge_requests/61