Add powershell support for docker executor
- 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
- 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.