ci: cover missing Dependabot dependency roots (#2612)

## Summary

- Add Dependabot Docker coverage for Docker Compose roots used by
root/package integration tests.
- Cover the root integration-test compose file, internal
environment-test compose file, and checkpoint package compose files.
- Keep existing GitHub Actions and root pnpm workspace coverage
unchanged.

## Test Plan

- [x] Parse `.github/dependabot.yml` with Ruby YAML.
- [x] Run `git diff --check`.
- [x] Inspect the diff to confirm it only changes
`.github/dependabot.yml`.

Co-authored-by: langsmith-fleet[bot] <langsmith-fleet[bot]@users.noreply.github.com>