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>

Dockerfile

@@ -0,0 +1,50 @@
+# Multi-stage Dockerfile for Next.js 16 static export
+
+# Stage 1: Dependencies
+FROM node:22-alpine AS deps
+RUN apk add --no-cache libc6-compat
+WORKDIR /app
+
+# Install pnpm
+RUN corepack enable && corepack prepare pnpm@latest --activate
+
+# Copy package files
+COPY package.json pnpm-lock.yaml ./
+
+# Install dependencies
+RUN pnpm install --frozen-lockfile
+
+# Stage 2: Builder
+FROM node:22-alpine AS builder
+WORKDIR /app
+
+# Install pnpm
+RUN corepack enable && corepack prepare pnpm@latest --activate
+
+# Copy dependencies from deps stage
+COPY --from=deps /app/node_modules ./node_modules
+
+# Copy source code
+COPY . .
+
+# Build the application (static export)
+RUN pnpm build
+
+# Stage 3: Runner (serve static files)
+FROM nginx:alpine AS runner
+
+# Copy custom nginx config
+COPY --from=builder /app/nginx.conf /etc/nginx/nginx.conf
+
+# Copy static files from build
+COPY --from=builder /app/out /usr/share/nginx/html
+
+# Expose port 80
+EXPOSE 80
+
+# Health check
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD wget --no-verbose --tries=1 --spider http://localhost/ || exit 1
+
+# Start nginx
+CMD ["nginx", "-g", "daemon off;"]