Docker build cache might be in the wrong order

[teor2345]

Motivation

The branch cache is likely to be faster and cheaper than the main cache, because the code is more similar:

zebra/.github/workflows/build-docker-image.yml

Lines 142 to 146 in 202dc43

	# We use multiple cache sources to confirm a cache hit, starting from the `main` branch cache,
	# and if there's no hit, then continue with a cache scoped per branch.
	cache-from: |
	type=registry,ref=us-docker.pkg.dev/zealous-zebra/zebra/${{ inputs.image_name }}:main-cache
	type=registry,ref=us-docker.pkg.dev/zealous-zebra/zebra/${{ inputs.image_name }}:${{ env.GITHUB_REF_SLUG_URL }}-cache

This is a medium priority, because we're trying to reduce costs.