Docker Compose interpolates all variables in the full file even when
only a subset of services is requested. The backend service uses
IMPORT_HOST_DIR with :? (hard-required), causing the idempotency job
to abort before any container starts. A dummy path satisfies the parser;
the backend service is never started in this job so the path need not exist.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>