chore: add Docker deployment behind Caddy

Build the static Next.js export in a reproducible container and expose it
through the existing Caddy network and upstream service name.

Dockerfile

@@ -0,0 +1,21 @@
+FROM node:22-alpine AS builder
+
+WORKDIR /app
+
+COPY package.json package-lock.json ./
+RUN npm ci
+
+COPY . .
+RUN npm run build
+
+FROM nginx:alpine AS runtime
+
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+COPY --from=builder /app/out/ /usr/share/nginx/html/
+
+EXPOSE 80
+
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD wget --no-verbose --tries=1 --spider http://127.0.0.1/ || exit 1
+
+CMD ["nginx", "-g", "daemon off;"]