Why is a prefix required when running in a single tenant setting:

4:11PM FTL failed to write schema error="rpc error: code = InvalidArgument desc = error parsing schema: parse error in `schema`, line 2, column 1: found reference `user` without prefix"

section of docker-compose.yml:

spicedb:
        container_name: spicedb
        image: authzed/spicedb:v1.43.0
        command: serve
        ports:
            - "50051:50051"
            - "8443:8443"
        environment:
            SPICEDB_GRPC_PRESHARED_KEY: testingkey
            SPICEDB_HTTP_ENABLED: true
            SPICEDB_DATASTORE_ENGINE: &spicedb_db_engine "postgres"
            SPICEDB_DATASTORE_CONN_URI: &spicedb_db_conn_uri "postgres://postgres:<pwd>@pg:5432/spicedb?sslmode=disable"
            SPICEDB_LOG_FORMAT: console