feat: add Docker support with GitHub Actions CI/CD

Add comprehensive Docker deployment with automated builds:

**Docker Configuration:**
- Multi-stage Dockerfile for optimized Next.js production builds
  - Stage 1: Install dependencies with pnpm
  - Stage 2: Build application with standalone output
  - Stage 3: Minimal runtime image with non-root user
  - Includes health check endpoint
  - Final image size optimized
- .dockerignore for efficient build context
- Enable standalone output in next.config.ts for Docker

**GitHub Actions Workflow:**
- Automated Docker image builds on push to main and tags
- Multi-platform support (linux/amd64, linux/arm64)
- Push to GitHub Container Registry (ghcr.io)
- Smart tagging strategy:
  - `latest` for main branch
  - `vX.X.X` for semver tags
  - `main-SHA` for commit-specific images
- Build cache optimization with GitHub Actions cache
- Artifact attestation for supply chain security

**Docker Compose:**
- Combined stack for UI + API
- Environment variable configuration
- Health checks for both services
- Automatic restart policies
- Shared network configuration

**Documentation:**
- Updated README with Docker deployment instructions
- Pre-built image usage from GHCR
- Docker Compose setup guide
- Local build instructions
- Available image tags reference

**Production Ready:**
- Images automatically published to ghcr.io/valknarness/pastel-ui
- Supports both x64 and ARM64 architectures
- Health checks for container orchestration
- Environment-based configuration
- Non-root user for security

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

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

.github/workflows/docker.yml

@@ -0,0 +1,72 @@
+name: Docker Build & Push
+
+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
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels)
+        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: .
+          platforms: linux/amd64,linux/arm64
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          build-args: |
+            NEXT_PUBLIC_API_URL=${{ vars.NEXT_PUBLIC_API_URL || 'http://localhost:3001' }}
+
+      - name: Generate artifact attestation
+        if: github.event_name != 'pull_request'
+        uses: actions/attest-build-provenance@v1
+        with:
+          subject-name: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          subject-digest: ${{ steps.build-and-push.outputs.digest }}
+          push-to-registry: true