Updated README for Dev and Beta channel container images -- added docker-compose files

[jamesmortensen]

• Updated the README to include information about the Dev/Beta release channel container images.
• Added sample docker-compose files for running a Grid of Dev channel browsers and Beta channel browsers.

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Description

Motivation and Context

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[diemol]

Thank you, @jamesmortensen!