ci: pull from Docker Hub directly, remove broken registry cache/mirrors

docker/Dockerfile

@@ -1,5 +1,5 @@
 # Base image: uses Harbor proxy cache in CI, falls back to Docker Hub externally
-FROM registry.celestium.life/dockerhub-cache/library/python:3.12-slim
+FROM python:3.12-slim
 
 ENV PYTHONDONTWRITEBYTECODE=1
 ENV PYTHONUNBUFFERED=1

docker/Dockerfile.scheduler

@@ -1,6 +1,6 @@
 # Scheduler image — adds postgresql-client for running migrations via psql.
 # Built separately from the base service image. Only the scheduler uses this.
-FROM registry.celestium.life/dockerhub-cache/library/python:3.12-slim
+FROM python:3.12-slim
 
 ENV PYTHONDONTWRITEBYTECODE=1
 ENV PYTHONUNBUFFERED=1

docker/Dockerfile.superset

@@ -1,6 +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
-FROM registry.celestium.life/dockerhub-cache/apache/superset:latest
+FROM apache/superset:latest
 
 USER root
 RUN /app/docker/pip-install.sh --no-cache trino[sqlalchemy] psycopg2-binary redis