ci: cover example Dockerfile with Dependabot (#681)

## Summary

- Add Dependabot Docker coverage for the maintained async subagent
server example Dockerfile.
- Leave npm coverage unchanged because the root pnpm workspace entry
already covers workspace package manifests through the root lockfile.
- Do not add release/N-1 `target-branch` entries; that policy applies
only to `langchainplus` and `langgraph-api`.

## Test Plan

- [x] Compared dependency roots on `main` against
`.github/dependabot.yml` coverage.
- [x] Parsed `.github/dependabot.yml` with Ruby YAML.
- [x] Ran `git diff --check`.

Co-authored-by: Langster <john@langchain.dev>