Add shellcheck to install-essentials
What does this MR do and why?
This brings a simpler version of !776 (closed) where we just add shellcheck
to the install-essentials
script.
Checklist
-
(If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.
Edited by Rémy Coutable