Add shellcheck for bash linting

[neilbmclaughlin]

With the introduction of bash scripts for docker and rancher on we should add shellcheck (https://www.shellcheck.net/) to our git hooks.

If we stick with the python scripts we will also need a python linter too.

This will apply to other repo's too.

[st3v3nhunt]

Closing the issue as it was originally created when the deployment scripts that are now part of nhsuk/ci-deployment were part of the repo.