Docker executor can lose stdout produced after a slow first script command

[pandasoft-zz]

Minimal .gitlab-ci.yml illustrating the issue

---
stages: [test]

rootless-image-job:
  stage: test
  image:
    name: moby/buildkit:rootless
    entrypoint: [""]
  script:
    - buildctl-daemonless.sh --version || true
    - mkdir -p dist
    - echo "rootless image job ok"
    - echo "rootless image job ok" > dist/rootless.txt
  artifacts:
    paths:
      - dist/rootless.txt

Expected behavior
The job should exit with status 0.

The gitlab-ci-local output should include:

rootless image job ok

The artifact dist/rootless.txt should also contain:

rootless image job ok

Actual behavior: the job exits with status 0, and the artifact is created with the expected content, but the stdout line is sometimes missing from the gitlab-ci-local process output.

Host information
Ubuntu 24.04
gitlab-ci-local 4.72.0

Containerd binary
Docker

Additional context
This seems to happen when a Docker job has a slow first script command. In this example, buildctl-daemonless.sh --version || true can take long enough to trigger the issue.

The later commands still run, because the artifact is created. Only stdout relay appears to be lost.

This makes stdout-based checks unreliable for Docker jobs that run a slow setup command before printing output.

From reading the runtime behavior, this may be related to how Docker jobs are attached. Docker start --attach --interactive attaches stdout/stderr and stdin. If the attach session ends too early after stdin closes, later container output may not be relayed even though the container continues and exits successfully.