Two live-test findings in one fix:
1. DockerExecutor has no repo fallback: when a project has no
repo_url, worker leaves task['repo'] unset and the container
host.py dispatch fails instantly with 'missing --repo' because
host-config-docker.json had no repo field either. Add the host
test-repo as the config default; projects with repo_url still
override it via worker's explicit --repo.
2. Worker discarded stderr from the final error_text, so every
container/host crash surfaced only as the generic 'host
execution failed' and was undebuggable from the task record.
When stdout is empty, error_text now keeps the stderr tail
(last 1000 chars, where the real error line lives).
host
87a2ad0066