fix: use BASE_REGISTRY arg to route all FROM pulls through Harbor cache

Default: docker.io (unchanged for external builds)
Woodpecker passes: registry.celestium.life/dockerhub-cache

docker/Dockerfile.superset

@@ -1,6 +1,7 @@
 # Custom Superset image with Trino, PostgreSQL, and Redis drivers
-ARG BASE_IMAGE=apache/superset:latest
-FROM ${BASE_IMAGE}
+# 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
 
 USER root
 RUN /app/docker/pip-install.sh --no-cache trino[sqlalchemy] psycopg2-binary redis