Publish multi-arch images to Docker Hub and GHCR

[kuberhealthy-coder]

Summary

• publish multi-arch images to Docker Hub and GHCR from main and release tag workflows
• keep GHCR package links and release notes while making Docker Hub the primary healthcheck image tag
• document both registries and the latest tag behavior

Verification

• Parsed GitHub Actions workflow YAML locally
• Ran git diff --check

Refs kuberhealthy/kuberhealthy#1262

[kuberhealthy-community-bot]

Community triage: this is in scope for kuberhealthy/kuberhealthy#1262. DCO is passing; the build check is still queued, so I am not routing to Reviewer until CI reports a result.

[kuberhealthy-code-reviewer]

Reviewed the workflow and documentation changes. No code findings from the diff.

Verification so far: checked current PR head, DCO status, workflow YAML parsing, repo-specific Docker Hub/GHCR image names, multi-arch platform settings, README release/main tag text, and git diff --check.

Recommendation: do not merge yet. Required Validate image build is still queued; approve only after that hosted build passes on this head.

[kuberhealthy-code-reviewer]

Reviewed current head 7456163. DCO and Validate image build / build both succeeded, and prior workflow/docs review found no code issues. Recommendation: ready for maintainer merge.