chore: add dockerfile.demo

scripts/Dockerfile.demo

+# Build frontend dist.
+FROM node:20-alpine AS frontend
+WORKDIR /frontend-build
+
+COPY . .
+
+WORKDIR /frontend-build/web
+
+RUN npm install -g pnpm
+RUN pnpm i --frozen-lockfile
+RUN pnpm build
+
+# Build backend exec file.
+FROM golang:1.24-alpine AS backend
+WORKDIR /backend-build
+
+COPY . .
+COPY --from=frontend /frontend-build/web/dist /backend-build/server/router/frontend/dist
+
+RUN go build -o memos ./bin/memos/main.go
+
+# Make workspace with above generated files.
+FROM alpine:latest AS monolithic
+WORKDIR /usr/local/memos
+
+RUN apk add --no-cache tzdata
+ENV TZ="UTC"
+
+COPY --from=backend /backend-build/memos /usr/local/memos/
+COPY ./scripts/entrypoint.sh /usr/local/memos/
+
+EXPOSE 5230
+
+# Directory to store the data, which can be referenced as the mounting point.
+RUN mkdir -p /var/opt/memos
+VOLUME /var/opt/memos
+
+ENV MEMOS_MODE="prod"
+ENV MEMOS_PORT="5230"
+
+ENTRYPOINT ["./entrypoint.sh", "./memos"]