fix(ci): use service token for dependency PRs

[fqjony]

Summary

• prefer DEPENDABOT_REVIEWER_TOKEN for generated Docker dependency PRs
• keep GH_TOKEN as fallback for existing workflow compatibility

Validation

• git diff --check
• ruby YAML parse for docker-dependency-updater.yml

[Copilot]

Pull request overview

Updates the Docker dependency updater GitHub Actions workflow to authenticate using a dedicated service token when available, while preserving the existing GH_TOKEN secret as a fallback to avoid breaking current setups.

Changes:

• Use secrets.DEPENDABOT_REVIEWER_TOKEN (fallback to secrets.GH_TOKEN) for peter-evans/create-pull-request@v8 authentication.
• Use the same token selection for gh pr merge via GH_TOKEN env var.

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