Migrate multi-arch Docker builds to GitHub Actions

[DeathGun44]

PR description

Following up on Discord discussion with Simon, this PR introduces a standalone GitHub Actions workflow (.github/workflows/docker-publish.yml) to handle multi-arch Docker builds and publishing, made by first understanding the architecture used by web3signer.

Architecture

• Native runners: ubuntu-latest (amd64) + besu-arm64 (arm64) — no QEMU emulation
• Digest handoff: push-by-digest=true during matrix build; manifest job assembles via docker buildx imagetools create with immutable @sha256 refs — correctly handles OCI indices on Docker 29's containerd image store
• Layer caching: GHA cache-from/cache-to scoped per arch; write-once on the test-load step, read-only on the push step
• Concurrency & permissions: cancellation group + explicit contents: read per job

Co-existence
No Gradle tasks were removed or modified. This workflow runs in parallel with develop.yml and draft-release.yml during the transition period. Follow-up PRs will deprecate the legacy pipeline once this is proven stable.

Fixed Issue(s)

fixes #9818

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• Considered documentation and added the doc-change-required label to this PR if updates are required — no doc changes required, CI-only workflow addition
• Considered the changelog and included an update if required — no changelog entry needed, internal CI infrastructure change
• For database changes considered compatibility — not applicable