Speed up Docker Rust builds with caching

[cafca]

Summary

• enable GitHub Actions cache for docker/build-push-action
• add BuildKit cache mounts for npm and Cargo registry/git/target data
• copy the Rust binary out of the cached target dir before the runtime stage

Validation

• git diff --check
• parsed .github/workflows/ci.yml as YAML
• docker buildx build --check -f backend/Dockerfile .