(#98) Adds Optional Docker Steps

[JPRuskin]

Description Of Changes

Adds steps that can be used to produce Docker containers. Currently matched very closely to what is required with chocolatey\choco.

Motivation and Context

We need steps to produce Docker containers.

Testing

1. WIndows
   1. Spin up a machine compatible with the LTSC 2016 Docker containers, with Docker available
   2. Using Chocolatey\Choco as the example, so download that repository
   3. Run .\Build.ps1 --Target Docker from the root of the repository
   4. Test the new image with something similar to docker run --rm chocolatey/choco:latest-windows choco.exe --version
2. Linux
   1. Spin up an Ubuntu machine with Docker, Mono, and Dotnet available
   2. Download Chocolatey\Choco
   3. Run sudo .\Build.sh --Target Docker
   4. Test the new image

Operating Systems Tested

• Windows 10
• Windows 2016
• Ubuntu 20.04

Change Types Made

• ~[ ] Bug fix (non-breaking change).~
• [x] Feature / Enhancement (non-breaking change).
• ~[ ] Breaking change (fix or feature that could cause existing functionality to change).~
• ~[ ] Documentation changes.~
• ~[ ] PowerShell code changes.~

Change Checklist

• ~[ ] Requires a change to the documentation.~
• ~[ ] Documentation has been updated.~
• ~[ ] Tests to cover my changes, have been added.~
• ~[ ] All new and existing tests passed?~
• ~[ ] PowerShell code changes: PowerShell v2 compatibility checked?~

Related Issue

Fixes #98

[gep13]

@JPRuskin thanks for getting this added!