* ci(s3tables): drop docker pre-pull from Lakekeeper job
The lakekeeper repro is pure Go against the local weed binary; the job
kept failing on Docker Hub timeouts pulling python:3 and localstack
images the test never runs. Also drop the stale python-in-docker
comments left from the old harness.
* ci(s3tables): serve python:3 from GHA cache in the STS job
Retried pulls still die when both mirror.gcr.io and registry-1.docker.io
are unreachable from the runner. Cache the saved image tarball under a
weekly key: an exact hit skips the registry entirely, a miss pulls fresh
and refreshes the cache, and a stale tarball from a previous week is the
fallback when Docker Hub is down.
* ci(spark): pre-pull the spark tag the test actually runs
The workflow warmed apache/spark:3.5.8 with retries while the
testcontainers setup runs apache/spark:3.5.1, so the real image was
pulled at test time with no retry at all.