fix: stabilize docker build workflow for digest pushes

[Mairuis]

Summary

• split the build path into two event-specific flows: PR builds load locally for verification, push/workflow_dispatch builds push by digest for publish
• remove concurrent type=docker + type=image exporting in push runs, which caused intermittent Docker Hub blob upload failures
• run verification with explicit matrix platform to avoid amd64/arm64 manifest mismatch during docker run

Why

Push runs were failing when Buildx exported local and registry outputs together, and verification could fail on mismatched runner/image platforms. This change keeps verification and publishing reliable while preserving Docker Hub + GHCR manifest publishing in the merge job.

[github-actions]

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