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>

next.config.ts

@@ -3,6 +3,9 @@ import type { NextConfig } from 'next';
 const nextConfig: NextConfig = {
   reactStrictMode: true,
 
+  // Enable standalone output for Docker
+  output: 'standalone',
+
   // React Compiler disabled for now (requires babel-plugin-react-compiler)
   // experimental: {
   //   reactCompiler: true,