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[Docker] Adding Docker build

See merge request formations/cpp/gettingstartedwithmoderncpp!12

This reverts commit 880f81bc.

Fix the script introduced in #2: it was not handling properly the case of a deleted file. The management is done in the pre-commit script: the Python script does not perform any checks upon the file.

Split some cells in the first notebooks to take advantage of the fact Xeus-Cling now supports multiple initialization of a same variable (this make some examples more readable as we show one trick at a time).

Notebooks 1 and 2 of InRealEnvironement have been handled. Demo files have been added for the file structure notebook.