fix: hardcode Harbor cache URLs in FROM lines to bypass Docker Hub rate limits

docker/Dockerfile.superset

@@ -1,7 +1,6 @@
 # Custom Superset image with Trino, PostgreSQL, and Redis drivers
 # Base image: uses Harbor proxy cache in CI, falls back to Docker Hub externally
-ARG BASE_REGISTRY=docker.io
-FROM ${BASE_REGISTRY}/apache/superset:latest
+FROM registry.celestium.life/dockerhub-cache/apache/superset:latest
 
 USER root
 RUN /app/docker/pip-install.sh --no-cache trino[sqlalchemy] psycopg2-binary redis