feat: reusable-docker-build-publish optional amd64

[erikburt]

Makes amd64 builds optional in the reusable-docker-build-publish workflow.

Changes

• Add build-amd64-image input to the workflow
• Validate that at least one of build-amd64-image and build-arm64-image is true
• Update logic to build the matrix JSON
  • Appends an entry to the initially empty JSON array, if the input/env-var is true
• Update image digests creation to use a bash array, appending if build option was true.

Testing

https://github.com/smartcontractkit/chainlink/actions/runs/23619781674?pr=21739

^ This tests:

• an amd64 only build
• an arm64 only build
• a build that disable both (errors)

---

DX-3203