Commit 3c8abb6f made docker-update.sh exit 0 (SKIP) on a host owned by CI so
the cron does not report a failed deployment. The tests still expected a
nonzero exit for the ci-active/ci-green/ci-upstream scenarios, so five cases
failed.
- docker-update.test.ts: move the CI scenarios out of the non-zero group and
assert the skip: exit 0, 'This host is managed by CI' in the output and no
'compose up'. All three CI scenarios (both slot containers and the nginx
upstream) now share one test.
- README.md: document the exit-0 skip.