Install shellcheck into Docker image

What does this Merge Request do?

I opted to install shellcheck directly via a GitHub release because the version available for ubuntu:18.04 was 0.4.6 where as the latest release is 0.7.1 😬

Related to !1190 (merged).

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• [-] This change is backward compatible. If not, please include steps to communicate to our users.
• [-] Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.