feat: add GitHub Actions workflow for Docker image builds

- Created docker-build.yml workflow for automated Docker builds
- Configured GitHub Container Registry (ghcr.io) with GITHUB_TOKEN auth
- Added multi-architecture support (linux/amd64, linux/arm64)
- Integrated Trivy security scanning for vulnerability detection
- Implemented smart tagging based on branches, PRs, and releases
- Added build caching for improved performance
- Updated Dockerfiles with OCI labels for proper ghcr.io linking
- Created comprehensive README-DOCKER.md documentation

The workflow automatically builds and pushes images on:
- Push to main/develop branches
- Pull requests (build only, no push)
- Release publications
- Manual workflow dispatch

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/Dockerfile

@@ -1,7 +1,15 @@
 FROM node:18-alpine AS builder
 
-# Add build argument for cache busting
+# Add build arguments
 ARG CACHEBUST=1
+ARG BUILD_DATE
+ARG VCS_REF
+ARG VERSION
+
+# Add labels for GitHub Container Registry
+LABEL org.opencontainers.image.source="https://github.com/the-luap/picpeak"
+LABEL org.opencontainers.image.description="PicPeak Backend Service"
+LABEL org.opencontainers.image.licenses="MIT"
 
 WORKDIR /app