Add CI/CD with Multi-arch Docker Builds and Cosign Signing

[bupd]

Summary

• Optimize Dockerfile with multi-stage build using Bun instead of Node.js
• Add GoReleaser config for multi-platform binary releases
• Add CI workflow for automated docker builds on push to main
• Add release workflow triggered on version tags
• Sign all docker images with cosign keyless signing via Sigstore

Docker Images

• Multi-arch support: linux/amd64, linux/arm64
• Published to ghcr.io/bupd/shitpost
• Signed with cosign for supply chain security

Binaries (on release)

• linux/amd64, linux/arm64
• darwin/amd64, darwin/arm64
• windows/amd64, windows/arm64