The frontend `e2e_test` doesn't have a working build cache setup,
causing really slow builds = slow test jobs. These changes reduce total
test runtime from ~12 minutes to ~5 minutes.
### Changes 🏗️
- Inject build cache config into docker compose config; let `buildx
bake` use GHA cache directly
- Add `docker-ci-fix-compose-build-cache.py` script
- Optimize `backend/Dockerfile` + root `.dockerignore`
- Replace broken DIY pnpm store caching with `actions/setup-node`
built-in cache management
- Add caching for test seed data created in DB
### Checklist 📋
#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- CI
