Create basic workflow for testing the local Docker environment.

[desrosj]

This creates a basic workflow that tests the local Docker environment to ensure that it runs smoothly when updated.

Eventually the PHPUnit tests will be moved out of the Docker containers (ticket coming soon) to benefit from better performance and more flexibility. When that happens, the container will no longer be tested regularly.

Trac ticket: https://core.trac.wordpress.org/ticket/62197

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[github-actions]

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[github-actions]

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Props desrosj, clorith.

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[desrosj]

@Clorith I gave up on getting Windows to work. It seems that MacOS and Windows runners cannot run Docker, or at least there's no good guides for how to accomplish that.

That makes this workflow a bit less useful. Ideally we can confirm everything works on the platforms that our contributors are using. But there's probably still some benefit to confirming that it works overall.