Create runner helper Docker image for PowerShell core

Description

Powershell Core isn't available for the following architectures that we build the GitLab Runner helper image on:

• alpine arm (only exists for ubuntu arm)
• arm64
• linux s390x

Proposal

• Create a GitLab Runner helper Docker image specifically for Powershell Core.
• This Powershell Core helper Docker image would be referenced whenever the job shell is pwsh.

Links to related issues and merge requests / references