GitLab Runner pwsh shell runs jobs as the root user on Linux OS

Summary

GitLab runner's pwsh shell runs as the root user on linux systems, not gitlab-runner user like the rest of the shell executors.

Steps to reproduce

  1. Spin up a fresh linux instance and install pwsh per Microsoft's docs for whatever flavor you're using
  2. Install gitlab-runner as per our own docs
  3. Register the runner on with whatever instance of GitLab you have available, I used GitLab.com and select the shell executor
  4. Edit the config.toml to specify pwsh as the shell
  5. Spin up a quick and simple CI job that runs whoami and observe
.gitlab-ci.yml

Technically the job below is not a failure, it complete successfully but with the unexpected result of showing that commands are being run as root

build-job:
  stage: build
  tags:
      - pwsh
  script:
    - whoami

Actual behavior

whoami returns the root user

Expected behavior

whoami return the gitlab-runner user

Environment description

[root@[redacted] ~]# hostnamectl
   Static hostname: [redacted]
         Icon name: computer-container
           Chassis: container
        Machine ID: 961dd5fd78de4e0d8842fc4134e94ebb
           Boot ID: e7af5ff8e583487caac8a639f8036853
    Virtualization: openvz
  Operating System: CentOS Linux 7 (Core)
       CPE OS Name: cpe:/o:centos:centos:7
            Kernel: Linux 3.10.0
      Architecture: x86-64
config.toml contents 
concurrent = 1
check_interval = 0

[session_server]
  session_timeout = 1800

[[runners]]
  name = "LinuxRunner"
  url = "https://gitlab.com/"
  token = [redacted]
  executor = "shell"
  shell = "pwsh"
  [runners.custom_build_dir]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]
    [runners.cache.azure]

Used GitLab Runner version

Version:      13.10.0
Git revision: 54944146
Git branch:   13-10-stable
GO version:   go1.13.8
Built:        2021-03-21T09:13:25+0000
OS/Arch:      linux/amd64

Possible fixes

Relevant logs and/or screenshots

job log 

Running with gitlab-runner 13.10.0 (54944146)
  on [redacted]
Resolving secrets 00:00
Preparing the "shell" executor 00:00
Using Shell executor...
Preparing environment 00:00
Running on linux...
Getting source from Git repository 00:02
Fetching changes with git depth set to 50...
Reinitialized existing Git repository in /home/gitlab-runner/builds/fZrTbjF9/0/JamesRLopes/powershell-testing/.git/
Checking out 381f6bd0 as master...
Removing misc/
Skipping Git submodules setup
Executing "step_script" stage of the job script 00:02
$ whoami
root
Cleaning up file based variables 00:00
Job succeeded
```

</details>
Edited by Darren Eastman