Update shellcheck in CI

[phlax]

description

Currently we use the shellcheck (v 0.4.6-1) provided by Ubuntu bionic in CI.

This is quite out of date - shellcheck has improved with each version, and its different to the versions found at least on ~recent linux desktops (not sure about mac/windows ?)

As #13106 lands all of the code will be brought under shellcheck coverage, and already some changes have landed which would have failed in a newer version, so it would be good in terms of maintaining the linting to upgrade this package.

I dont see any ~official PPA with a backport - so i guess one of

• using upstream directly
• using a docker image

...will be the easiest way to upgrade

refs

• https://www.ubuntuupdates.org/ppas
• https://github.com/koalaman/shellcheck
• https://hub.docker.com/r/koalaman/shellcheck