Add Docker Build Caching in GitHub Actions

[wei]

Add caching to docker.yml workflow. We currently use multi-stage builds.

• https://pnpm.io/docker - Look into BuildKit cache and cache mounts.
• https://github.com/docker/build-push-action
• https://docs.docker.com/build/ci/github-actions/cache/ - Inline cache doesn't seem to work with multi-stage builds (pls correct me if I'm wrong). perhaps look into GitHub Cache API.

Contributions welcome, please comment below if interested. 🫶