Add powershell support for docker executor

Description

⚠ Blocked by: #3755 (closed), #3895 (closed) ⚠

When we stat supporting multiple helper images for windows, we need to update the init() so that it can use the windows version.

Proposal

1. Switch the base image for helper image to server core.
   • Yes, this will make the helper image 4.5GB but the user will already have the large layers locally anyways because their jobs are most likely using server core (as explained in #3896 (comment 153848415)
   • The size of the container image is not that big of an issue since we do not support loading the image from a local archive as we do for Linux
2. By switching to servercore we can use PowerShell 5 for now, making the first iteration even smaller, since we don't have to support PowerShell 6 and as described above we don't have to handle switching BOM on and off depending on what PowerShell version we use.

For now we are only going to support PowerShell 5 when using the windows docker executor.

We can do this in multiple ways.

1. We can use the shell configuration form the [[runner]] section 1. We can detect the runtime of the runner with runtime.GOOS

Links to related issues and merge requests / references

Blocked by: #3755 (closed) Epic: &535 (closed)