workflows/docker: stop pushing to Docker Hub, remove 20.04 image

[p-linnane]

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We've decided to stop pushing to Docker Hub due to reliability issues. This PR also removes the 20.04 image in preparation for Homebrew 4.7.0.

[Copilot]

Pull Request Overview

This PR removes Docker Hub as a publishing target for Homebrew's Docker images and drops support for Ubuntu 20.04 images in preparation for Homebrew 4.7.0 release. The changes streamline the Docker workflow by eliminating deprecated image variants and simplifying the publishing pipeline.

Key changes:

• Removed Docker Hub authentication and all homebrew/* image tags
• Eliminated Ubuntu 20.04 from the version matrix and removed related conditional logic
• Cleaned up deprecation notices and special handling for 20.04 images

Comments suppressed due to low confidence (1)

.github/workflows/docker.yml:1

• With the removal of the 20.04 version check, this line will now attempt to add an ARM64 digest for all versions. However, the code on line 278 initializes image_args as a single-element array. This line should unconditionally add the ARM64 digest since 20.04 (which had no ARM64 build) has been removed from the version matrix.

name: Docker

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[MikeMcQuaid]

Looks good! Marking draft just so we don't merge before we're ready for 4.7 being next release.