Fix path substitution in the hbck2 script

[siegfriedweber]

Description

The ShellCheck comment turned the whole RUN argument into a comment and the paths in the hbck2 script were not substituted. This was the reason why no ShellCheck warning was emitted. Disabling the ShellCheck warning does not work in a Dockerfile, because there can be no separate line in the command which disables the ShellCheck, and inline comments with `# ...` and $(: ...) are not recognized by ShellCheck.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

- [x] Changes are OpenShift compatible
- [x] All added packages (via microdnf or otherwise) have a comment on why they are added
- [x] Things not downloaded from Red Hat repositories should be mirrored in the Stackable repository and downloaded from there
- [x] All packages should have (if available) signatures/hashes verified
- [x] Does your change affect an SBOM? Make sure to update all SBOMs