Add Docker support and update README

- Add Dockerfile with multi-stage build for Node.js + React
- Add docker-compose.yml for easy local deployment
- Add .dockerignore to exclude unnecessary files
- Add GitHub Actions workflow for automatic Docker image builds and pushes to GHCR
- Add comprehensive DOCKER.md documentation
- Update README with Docker installation instructions

Dockerfile

@@ -0,0 +1,52 @@
+# Build stage for React client
+FROM node:20-alpine AS client-builder
+
+WORKDIR /app/client
+
+# Copy client package files
+COPY client/package*.json ./
+
+# Install dependencies
+RUN npm ci
+
+# Copy client source
+COPY client/src ./src
+COPY client/public ./public
+
+# Build React app
+RUN npm run build
+
+# Final stage - Node.js server
+FROM node:20-alpine
+
+WORKDIR /app
+
+# Install dumb-init for proper signal handling
+RUN apk add --no-cache dumb-init
+
+# Copy server package files
+COPY package*.json ./
+
+# Install production dependencies only
+RUN npm ci --only=production
+
+# Copy server code
+COPY server ./server
+
+# Copy built React client from builder stage
+COPY --from=client-builder /app/client/build ./client/build
+
+# Create non-root user
+RUN addgroup -g 1001 -S nodejs && \
+    adduser -S nodejs -u 1001
+
+USER nodejs
+
+# Expose port
+EXPOSE 3001
+
+# Use dumb-init to handle signals properly
+ENTRYPOINT ["dumb-init", "--"]
+
+# Start the server
+CMD ["node", "server/index.js"]