ci: add Docker build and GitHub Container Registry workflow

Add complete Docker containerization and CI/CD setup:

Docker Configuration:
- Multi-stage Dockerfile with 3 stages (deps, builder, runner)
- Stage 1: Install dependencies with pnpm in Alpine
- Stage 2: Build Next.js static export
- Stage 3: Serve static files with nginx:alpine
- Health check endpoint on /health
- Optimized for production with layer caching

Nginx Configuration:
- Custom nginx.conf for static file serving
- Gzip compression enabled
- Security headers (X-Frame-Options, X-Content-Type-Options, etc.)
- Static asset caching with 1-year expiry
- Client-side routing support (try_files)
- Health check endpoint for container orchestration
- Error page handling

GitHub Workflow (docker-build-push.yml):
- Triggers on push to main, tags, and pull requests
- Multi-platform builds (linux/amd64, linux/arm64)
- Pushes to GitHub Container Registry (ghcr.io)
- Automatic tagging: latest, semver, sha, branch
- Uses GitHub Actions cache for faster builds
- Requires only GITHUB_TOKEN (no secrets needed)

.dockerignore:
- Excludes node_modules, .next, build artifacts
- Excludes dev files, logs, and IDE configs
- Keeps Docker image size minimal

Image will be available at:
ghcr.io/valknarness/units-ui:latest

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

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

.github/workflows/docker-build-push.yml

@@ -0,0 +1,64 @@
+name: Build and Push Docker Image
+
+on:
+  push:
+    branches:
+      - main
+    tags:
+      - 'v*'
+  pull_request:
+    branches:
+      - main
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=sha,prefix={{branch}}-
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          platforms: linux/amd64,linux/arm64
+
+      - name: Image digest
+        run: echo ${{ steps.meta.outputs.digest }}