docker: use public base images for portal builds

Dockerfile.backend

@@ -1,4 +1,4 @@
-FROM registry.bstein.dev/bstein/python:3.12-slim-arm64
+FROM python:3.12-slim
 
 ENV PYTHONDONTWRITEBYTECODE=1 \
     PYTHONUNBUFFERED=1

Dockerfile.frontend

@@ -1,5 +1,5 @@
 # Build stage
-FROM registry.bstein.dev/bstein/node:20-alpine-arm64 AS build
+FROM node:20-alpine AS build
 WORKDIR /app
 
 COPY frontend/package*.json ./
@@ -10,7 +10,7 @@ COPY media/ ./public/media/
 RUN npm run build
 
 # Runtime stage
-FROM registry.bstein.dev/bstein/nginx:1.27-alpine-arm64
+FROM nginx:1.27-alpine
 WORKDIR /usr/share/nginx/html
 
 # Minimal nginx config with SPA fallback.