GitLab Unix shell scripting style guide

## Description of the proposal Across GitLab, we use pretty much of Unix **shell scripting** (`sh`, `bash` and maybe something else). But we don't have an appropriate style guide yet. This is a proposal to: - create a blueprint of such a style guide - discuss and contribute to the contents - adopt it and add to the documentation - would be great to also find and adopt linters to check the compliance in future - [x] Mention the proposal in the next backend weekly call and the #backend channel to encourage contribution - [ ] Proceed with the proposal once 50% of the maintainers have weighed in, and 80% of the votes are :+1: - [ ] Once approved, mention it again in the next backend weekly call and the #backend channel /cc @gitlab-org/maintainers/rails-backend ## Good examples - https://google.github.io/styleguide/shell.xml - more? ## Development Log ### Status - [x] Prepare an MVC and push for review - [x] Collect the required amount of feedback and votes - [x] Polish before the merge and final announcement - [x] Merge and announce before the GitLab backenders ### Decisions - As for an MVC, it's decided to go for [ShellCheck](https://github.com/koalaman/shellcheck) and take the [shfmt](https://github.com/mvdan/sh#shfmt) into consideration.

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