Update the docker image to use doxec version 0.2.0 with improved user interface.

Use CI image with an unprivileged user student, which has no-password sudo
powers, such that the installation snipped from the README.md can be executed
directly.

Remove superuser privileges from pip and install packages in ~/.local. This also
prevents hiding pip's packages by system packages, since user packages have
precedence.

Add a CI pipeline with a single doxec task. The created python code examples are
packaged and can be downloaded on success.