Add Dependabot configuration for GitHub Actions

[ScottBrenner]

Noticed the actions used in the workflows here are outdated, proposing a Dependabot configuration to update - reference https://docs.github.com/en/actions/security-guides/using-githubs-security-features-to-secure-your-use-of-github-actions#keeping-the-actions-in-your-workflows-secure-and-up-to-date

Resolves warnings on workflow runs e.g. https://github.com/kernel/kernel-images/actions/runs/30600123889

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4

Suggest enabling https://docs.github.com/en/code-security/dependabot/working-with-dependabot/about-dependabot-on-github-actions-runners#enabling-or-disabling-for-your-repository as well

Checklist

• A link to a related issue in our repository
• A description of the changes proposed in the pull request.
• @mentions of the person or team responsible for reviewing proposed changes.

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Note

Low Risk
Configuration-only change with no runtime or application code impact; Dependabot PRs will still need normal CI review before merging.

Overview
Adds Dependabot via .github/dependabot.yml so third-party GitHub Actions pins in workflow files under the repo root are checked weekly and can be bumped through Dependabot PRs.

This does not change any workflow logic or action versions in this diff; it only enables automated update proposals for dependencies such as actions/checkout@v4 and the Docker build actions used in CI.

^{Reviewed by Cursor Bugbot for commit 439b9cd. Bugbot is set up for automated code reviews on this repo. Configure here.}

[ScottBrenner]

Hey @sjmiller609 spare a moment to review?