Split tags per registry in image merge step

[yubiuser]

What does this PR aim to accomplish?:

Same as pi-hole/docker-pi-hole#2005.

After updating buildx to 0.31.1 our image merge workflow broke.

After a very good discussion here with the maintainers of buildx, it turned out, that our workflow always just worked due to a bug.
During the discussion, a workaround was suggested by splitting the tags on per-registry basis. This is what this PR implements. This is based on a suggestion by crazy-max.

This supersedes #140

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[yubiuser]

Superseded by #152