Our Dockerfile is hardly used, but it does still hold value. This
commit bumps the version of the Git package, to a supported version.
Also removes the cloned version of GitLab-Shell, as the required parts
are absorbed already now.
Ruby 2.6 offers better performance and memory usage:
https://www.rubyguides.com/2018/11/ruby-2-6-new-features/
Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/57323