Fix production deployment: replace serve with nginx reverse proxy

Frontend container now uses nginx to serve static files and proxy
/api/* requests to the backend container internally, eliminating
the hardcoded localhost:8000 build-time URL that caused "Network
error" on any non-local server. CORS origins are also configurable
via ALLOWED_ORIGINS env var.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

btc-portfolio/frontend/Dockerfile

@@ -4,16 +4,12 @@ WORKDIR /app
 COPY package.json ./
 RUN npm install
 COPY . .
-ARG REACT_APP_API_URL=http://localhost:8000
+ARG REACT_APP_API_URL=/api
 ENV REACT_APP_API_URL=$REACT_APP_API_URL
 RUN npm run build
 
-FROM node:18-alpine
-RUN npm install -g serve
-RUN addgroup -S appgroup && adduser -S appuser -G appgroup
-WORKDIR /app
-COPY --from=build /app/build ./build
-RUN chown -R appuser:appgroup /app
-USER appuser
-EXPOSE 3001
-CMD ["serve", "-s", "build", "-l", "3001"]
+FROM nginx:alpine
+COPY --from=build /app/build /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80
+CMD ["nginx", "-g", "daemon off;"]