fix: use BASE_IMAGE build args to pull through Harbor cache, avoid Docker Hub rate limits

Dockerfiles default to Docker Hub images (unchanged for external builds).
Woodpecker passes registry.celestium.life/dockerhub-cache/... via build args.

docker/Dockerfile

@@ -1,4 +1,5 @@
-FROM python:3.12-slim
+ARG BASE_IMAGE=python:3.12-slim
+FROM ${BASE_IMAGE}
 
 ENV PYTHONDONTWRITEBYTECODE=1
 ENV PYTHONUNBUFFERED=1

docker/Dockerfile.superset

@@ -1,5 +1,6 @@
 # Custom Superset image with Trino, PostgreSQL, and Redis drivers
-FROM apache/superset:latest
+ARG BASE_IMAGE=apache/superset:latest
+FROM ${BASE_IMAGE}
 
 USER root
 RUN /app/docker/pip-install.sh --no-cache trino[sqlalchemy] psycopg2-binary redis