feat: implement full CI/CD pipeline with GitHub Actions and Flux

- Add GitHub Actions workflow to build and push Docker images to GHCR
- Add Dockerfile for multi-stage builds
- Update deployment to use GHCR images
- Add GHCR authentication secrets
- Configure Flux ImageRepository, ImagePolicy, and ImageUpdateAutomation
- Remove init container approach in favor of proper Docker builds

Auto-deploy flow:
1. Push to master triggers GitHub Actions
2. GitHub Actions builds image with commit hash tag
3. Image pushed to ghcr.io/unchainedio/manoon-headless
4. Flux ImageRepository detects new image
5. Flux ImageUpdateAutomation updates kustomization.yaml
6. Flux Kustomization applies new deployment
7. Kubernetes restarts pods with new image

k8s/kustomization.yaml

@@ -1,6 +1,9 @@
 apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 resources:
-  - deployment-nodejs.yaml
+  - deployment.yaml
   - service.yaml
   - ingress.yaml
+images:
+  - name: ghcr.io/unchainedio/manoon-headless
+    newTag: 2c27fc6  # Updated by GitHub Actions