feat: enhance Dockerfile and entrypoint for user permissions and environment variable handling

Dockerfile

@@ -9,6 +9,10 @@ RUN npm run build
 # Final runtime image
 FROM node:24-alpine
 WORKDIR /app
+
+# ensure su-exec present to drop privileges later
+RUN apk add --no-cache ca-certificates su-exec
+
 # copy package manifests and install deps (leverage Docker cache)
 COPY backend/package*.json backend/package-lock*.json ./backend/
 RUN cd backend && npm ci --silent