Shell executor build (not only run the service) as root user

Summary

I have installed and registered successfully an specific runner (10.1.0) to my GitLab instance (10.1.4). The runner use the shell executor. I placed a whoami inside the YAML to check the user running. AFAIK the runner starts the service as root user but the build should be made by the user (in my case gitlab-runner).

Strange: Sometimes the whoami shows root and sometimes gitlab-runner 😳

Steps to reproduce

Just retry the job several times and take a look into the log.

Actual behavior

Sometimes the whoami displays root and sometimes gitlab-runner.

Expected behavior

The whoami should always display gitlab-runner.

Used GitLab Runner version

Running with gitlab-runner 10.1.0 (c1ecf97f) and using Shell executor.