chore: add Dockerfile for TanStack Start production build

Dockerfile

@@ -0,0 +1,21 @@
+# Stage 1 — deps + build
+FROM oven/bun:1 AS builder
+WORKDIR /app
+
+COPY package.json bun.lock ./
+RUN bun install --frozen-lockfile
+
+COPY . .
+RUN bun run build
+
+# Stage 2 — production runtime
+FROM node:22-alpine AS runner
+WORKDIR /app
+
+COPY --from=builder /app/.output ./.output
+
+EXPOSE 3000
+
+ENV NODE_ENV=production
+
+CMD ["node", ".output/server/index.mjs"]