PR Summary

This allows for developing in a container. Once .Net Core 3 releases its RTM version, I would like to upgrade the SDK, which will then contain PowerShell already. At the moment I have to bootstrap PowerShell myself, which is at least easier then using the PowerShell extension and bootstrapping the .Net SDK.
Dockerfile taken from .Net Core template of Docs page for VS-Code container development.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
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