fix: use single registry for push-by-digest to fix manifest merge

[Mairuis]

Summary

• Fix the merge job failing with not found when creating manifest lists for Docker Hub
• Remove the second name=ghcr.io/... from the build job's push-by-digest output, so digests are only pushed to Docker Hub
• Update the GHCR manifest creation step to reference digests from Docker Hub instead of GHCR

Root Cause

The build job specified two name values in a single type=image output:

type=image,name=langgenius/dify-agentbox,name=ghcr.io/langgenius/dify-agentbox,push-by-digest=true,...

In push-by-digest mode with name-canonical=true, buildx only pushed the image layers to GHCR (the second name), not Docker Hub. The merge job then tried to reference these digests on Docker Hub and failed with not found.

Fix

Follow the Docker official docs pattern: push-by-digest to a single registry (Docker Hub), then use imagetools create in the merge job to create manifest lists for both Docker Hub and GHCR from the same source digests.

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