pre-commit: Add shellcheck to pre-commit configuration.

`Shellcheck` is a linter tool for shell scripts that is also used in
Automation Hub.

This change adds a pre-commit hook to run shellcheck on shell scripts.
The hook uses a Docker image, which needs to be downloaded on the first
run. It works well  with `podman`.

.pre-commit-config.yaml

@@ -38,3 +38,8 @@ repos:
     entry: utils/ansible-doc-test
     # args: ['-v', 'roles', 'plugins']
     files: ^.*.py$
+- repo: https://github.com/koalaman/shellcheck-precommit
+  rev: v0.8.0
+  hooks:
+  - id: shellcheck
+    args: ["--severity=warning"]  # Only show errors and warnings