Tezt/Cloud: Fix dockerfile UX

François Thirérequested to merge
francois@tezt-cloud-dockerfile-ux into master

What

Ensure tezt-cloud users can use individual files but we can still commit OS specific dockerfile for tezt-cloud.

Why

On !15393 (merged) fixes an old idiosyncrasy of tezt-cloud where the canonical dockerfile was an individual file from me instead of a canonical one that would be OS specific. However, it is important for users to be able to have their own dockerfile without committing them on the monorepo.

How

Via the .gitignore file, we ensure OS specific dockerfiles can still be committed while individual files won't. We update the README accordingly (and also fixes some few minors things).

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