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>

README.md

@@ -356,14 +356,55 @@ vercel --prod
 
 ### Docker
 
+#### Using Pre-built Image from GHCR
+
 ```bash
-# Build
+# Pull the latest image
+docker pull ghcr.io/valknarness/pastel-ui:latest
+
+# Run the container
+docker run -p 3000:3000 \
+  -e NEXT_PUBLIC_API_URL=http://localhost:3001 \
+  ghcr.io/valknarness/pastel-ui:latest
+```
+
+#### Docker Compose (UI + API)
+
+Run both Pastel UI and Pastel API together:
+
+```bash
+# Using docker-compose
+docker-compose up -d
+
+# View logs
+docker-compose logs -f
+
+# Stop
+docker-compose down
+```
+
+#### Building Locally
+
+```bash
+# Build the image
 docker build -t pastel-ui .
 
-# Run
-docker run -p 3000:3000 -e NEXT_PUBLIC_API_URL=https://api.pastel.com pastel-ui
+# Run locally built image
+docker run -p 3000:3000 \
+  -e NEXT_PUBLIC_API_URL=http://localhost:3001 \
+  pastel-ui
 ```
 
+#### Available Docker Images
+
+Images are automatically built and published to GitHub Container Registry:
+
+- `ghcr.io/valknarness/pastel-ui:latest` - Latest main branch
+- `ghcr.io/valknarness/pastel-ui:v1.0.0` - Specific version
+- `ghcr.io/valknarness/pastel-ui:main-abc1234` - Commit SHA
+
+Supported platforms: `linux/amd64`, `linux/arm64`
+
 ### Static Export
 
 ```bash