fix: Use folder commmit for Stackbrew

[nschonni]

Description

Current script just takes the commit hash for the Dockerfile, so when the commit bit was fixed, it isn't getting picked up for the upstream PR. Now it will use the whole variant folder when looking for the latest commithash

Motivation and Context

Testing Details

Example Output(if appropriate)

Types of changes

• Documentation
• Version change (Update, remove or add more Node.js versions)
• Variant change (Update, remove or add more variants, or versions of variants)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Others (non of above)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• All new and existing tests passed.

[github-actions]

Created PR on the official-images repo (docker-library/official-images#10748). See https://github.com/docker-library/faq#an-images-source-changed-in-git-now-what if you are wondering when it will be available on the Docker Hub.