feat: add multi-stage Dockerfile for PWA

Build stage uses node:20-alpine to install deps and build.
Prod stage uses nginx:stable-alpine to serve static assets.
Includes nginx config with SPA routing, gzip, health endpoint,
and aggressive caching for Vite-hashed assets.

Closes #6

Co-Authored-By: Paperclip <noreply@paperclip.ing>

Dockerfile

@@ -0,0 +1,21 @@
+# Stage 1: Build
+FROM node:20-alpine AS build
+
+WORKDIR /app
+
+COPY package.json package-lock.json ./
+RUN npm ci
+
+COPY . .
+RUN npm run build
+
+# Stage 2: Production
+FROM nginx:stable-alpine AS prod
+
+COPY --from=build /app/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+
+EXPOSE 80
+
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD wget -qO- http://localhost/health || exit 1