docker: review fixes from the 8-reviewer pass and staging CI

entrypoint.sh: a container started without a GPU request has no
nvidia-smi at all (the toolkit injects it), so the old check 1 reported
'CUDA runtime in this image is broken, re-pull' for the most common user
error. Fold the missing-binary case into the actionable 'No GPU visible'
message and document the CPU-only option (UNSLOTH_ALLOW_CPU=1).

run.sh / test_locally.sh: guard empty-array expansions with the
${arr[@]+...} form; bash 3.2 (macOS /bin/bash) treats "${empty[@]}"
as unbound under set -u, which broke the documented macOS CPU path.

studio_launch.sh: exclude *_TOKEN, *_API_KEY, *_PASSWORD, *_SECRET,
*_LICENSE from the env snapshot written for SSH sessions; secrets stay
in process env only, never on disk.

supervisord.conf / Dockerfile.studio: pin HOME=/root for the studio and
jupyter programs (jupyter would silently fall back to token auth if HOME
were unset), default JUPYTER_PORT and UNSLOTH_ENABLE_SSHD at the image
level so a direct supervisord invocation cannot hit a bad %(ENV_*)s
expansion, and document the root-services decision (non-root parity with
the previous production image is a tracked follow-up).

docker_confirm.ps1: mirror the bash script's GPU selector translation so
GPUS=0 / 0,1 select devices instead of silently using all GPUs.

docker-publish.yml: studio cache scope moves to mode=min; a mode=max
cache of a ~24GB image would evict everything else in the 10GB GHA
quota for no hit-rate gain.

docker/Dockerfile.studio

@@ -35,8 +35,19 @@ FROM ${BASE_IMAGE}
 ARG UNSLOTH_STUDIO_REF=main
 ARG TARGETARCH
 
+# Services run as root in this revision (the base image is root-only by
+# design); the previous production image ran them as a dedicated uid-1001
+# user. Non-root parity is a tracked follow-up. sshd is key-only and stays
+# disabled unless a PUBLIC_KEY/SSH_KEY is provided, and no secrets are
+# persisted to disk (see studio_launch.sh).
+#
+# The JUPYTER_PORT / UNSLOTH_ENABLE_SSHD defaults exist so supervisord's
+# %(ENV_*)s expansions still resolve when someone bypasses the launcher
+# and runs supervisord directly.
 USER root
 ENV UNSLOTH_STUDIO_HOME=/opt/unsloth-studio \
+    JUPYTER_PORT=8888 \
+    UNSLOTH_ENABLE_SSHD=false \
     DEBIAN_FRONTEND=noninteractive
 
 # install.sh needs curl + git; supervisor + openssh-server run the service