[GitHub Actions] Introduce Shellcheck to check bash code

[andygrunwald]

✍️ Description

ShellCheck finds bugs in your shell scripts. It is an open source tool (https://github.com/koalaman/shellcheck) that runs analysis on your shell scripts.

As this project uses shell and bash intensively, automated checks that help humans avoid bugs may be helpful.

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🛠️ Type of Change

Please check the relevant options:

• Bug fix (non-breaking change that resolves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change unexpectedly)
• New script (a fully functional and thoroughly tested script or set of scripts)

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✅ Prerequisites

The following steps must be completed for the pull request to be considered:

• Self-review performed (I have reviewed my code to ensure it follows established patterns and conventions.)
• Testing performed (I have thoroughly tested my changes and verified expected functionality.)
• Documentation updated (I have updated any relevant documentation)

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📋 Additional Information (optional)

This PR might not be mergable from the get go as shellcheck throws a lot of errors on the repository itself.

Sometimes it throws "false positives" due to the way the scripts are written.
E.g. DISK${i}_REF vs. DISK1_REF:

In vm/ubuntu2404-vm.sh line 400:
  -scsi0 ${DISK1_REF},${DISK_CACHE}${THIN}size=2G \
         ^----------^ SC2086 (info): Double quote to prevent globbing and word splitting.
         ^----------^ SC2153 (info): Possible misspelling: DISK1_REF may not be assigned. Did you mean DISK_REF?

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A full list of error codes and descriptions can be found here: https://www.shellcheck.net/wiki/

Those can be disables via

env:
    SHELLCHECK_OPTS: -e SC2059 -e SC2034 -e SC1090

[andygrunwald]

Shellcheck itself can't run right now here, due to https://github.com/community-scripts/ProxmoxVE/actions/runs/12502825499

ludeeus/action-shellcheck@master is not allowed to be used in community-scripts/ProxmoxVE. Actions in this workflow must be: within a repository owned by community-scripts or created by GitHub.