firecow/gitlab-ci-local: testing artifacts fails

.gitlab-ci-local-env

NEEDS=true
CLEANUP=true
PRIVILEGED=true
MOUNT-CACHE=true
VOLUME="/var/run/docker.sock:/var/run/docker.sock"
EXTRA-HOST="host.docker.internal:host-gateway"

.gitlab-ci.yml

include:

stages:
  - build

build-stage:
  stage: build
  artifacts:
    #expire_in: 15 minutes
    name: "$CI_JOB_STAGE-$CI_COMMIT_REF_NAME"
    paths:
      - /build/*
  script:
    - docker build --progress plain -f Dockerfile.artifact .

Dockerfile.artifact

# syntax = docker/dockerfile:1.4.0
FROM debian:stable-slim AS artifact-test

WORKDIR /build

RUN \
  echo "Test" > /build/my-artifact.txt \
  cat /build/my-artifact.txt