build: make embedded pi images reproducible

docker/frontend.Dockerfile

@@ -1,5 +1,6 @@
 # syntax=docker/dockerfile:1.7
 # thothii-frontend: build Vite (React) + nginx-unprivileged (porta 8080).
+ARG IMAGE_VERSION=local
 FROM node:22-bookworm AS build
 WORKDIR /src
 COPY frontend/package*.json ./
@@ -9,6 +10,10 @@ ENV VITE_BASE=/ VITE_BACKEND_URL=/api
 RUN npm run build
 
 FROM nginxinc/nginx-unprivileged:1.27-alpine AS runtime
+ARG IMAGE_VERSION
+LABEL org.opencontainers.image.title="thothii-frontend" \
+      org.opencontainers.image.version="${IMAGE_VERSION}" \
+      org.opencontainers.image.description="ThothII standalone frontend"
 COPY --from=build /src/dist /usr/share/nginx/html
 COPY docker/nginx.conf.template /etc/nginx/templates/default.conf.template
 COPY --chmod=755 docker/frontend-entrypoint.sh /usr/local/bin/frontend-entrypoint