fix: container diff improvements

Closes #17 (closed)

Related to https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/5513

The container-diff tool is deprecated/archived. https://github.com/GoogleContainerTools/container-diff

This removes it from the common-ci-tasks toolchain, switching to docker to calculate the size of the before and after images.

It also adds a test for this include, and updates the documentation accordingly.