Linting: Shellcheck

Continuing the effort started in !32 (merged) by adding a pipeline job for shellcheck (task listed in #11).

This MR is based on the decisions that have been made in !32 (merged) (rules, separate job for shellcheck, using alpine as image, etc...).
It'll also have to be rebased against !32 (merged) once the latter is merged.

Here's a run of the shellcheck job.
I'd happily help to fix/treat the "issues" raised by shellcheck once this MR is merged!

I remain available if any changes are needed