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>

docker-compose.yml

@@ -0,0 +1,43 @@
+version: '3.8'
+
+services:
+  pastel-ui:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    image: ghcr.io/valknarness/pastel-ui:latest
+    container_name: pastel-ui
+    ports:
+      - "3000:3000"
+    environment:
+      - NODE_ENV=production
+      - NEXT_PUBLIC_API_URL=${NEXT_PUBLIC_API_URL:-http://localhost:3001}
+      - NEXT_PUBLIC_APP_URL=${NEXT_PUBLIC_APP_URL:-http://localhost:3000}
+    restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "node", "-e", "require('http').get('http://localhost:3000/', (r) => {process.exit(r.statusCode === 200 ? 0 : 1)})"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 40s
+
+  pastel-api:
+    image: ghcr.io/valknarness/pastel-api:latest
+    container_name: pastel-api
+    ports:
+      - "3001:3001"
+    environment:
+      - RUST_LOG=info
+      - PORT=3001
+      - HOST=0.0.0.0
+    restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:3001/api/v1/health"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 10s
+
+networks:
+  default:
+    name: pastel-network