Fix Debian-based Docker image `org.opencontainers.image.source` label

[justusbunsi]

This should hopefully™️ help retrieving correct changelog information in Renovate PRs such as https://github.com/jenkinsci/helm-charts/pull/1135. It also fixes broken source link in the PR table.

Testing done

Not tested yet. Based on Renovate documentation, this URL must be correct, so that it can detect the changelog.

Submitter checklist

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or Jira
• [x] Link to relevant pull requests, esp. upstream and downstream changes
• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue