Add Dockerfile and nginx config for Coolify deployment

Multi-stage build: Node.js 22 for Astro build, nginx:alpine for serving.
Includes gzip, security headers, and immutable cache for _astro/ assets.

Dockerfile

@@ -0,0 +1,13 @@
+# Stage 1: Build
+FROM node:22-alpine AS build
+WORKDIR /app
+COPY package*.json ./
+RUN npm ci
+COPY . .
+RUN npm run build
+
+# Stage 2: Serve
+FROM nginx:alpine
+COPY --from=build /app/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80