fix multiarch buildah command

[Qusic]

follow up fix for #2757 😳

It seems the newly published images are single-arch. Went through a few manuals and it turned out that we should use --manifest instead of --tag.
Also buildah doesn't supports more than one --manifest arguments in one build, so we have to build each (v5, v5.15, v5.15.0, etc) separately even if they are identical images/manifests.

Ideally we can move downloadAssets.sh's logic into Containerfile itself so we can build for multiple archs concurrently. However that would be a big change and I'd rather leave it as is for now.

[xiaokangwang]

I think this merge request is ready to be merged.

I might rewrite this in the future to improve it. For now, since we are not building anything in the container, it should be fine even we are doing things sequentially.

BTW: we couldn't move download to the container, as we are building cross building without emulator, the RUN command is not available.

[Qusic]

@xiaokangwang please help merge this. I don't have write access to this repo and cannot merge it myself. Thanks!

As for future improvements, I suggest we use workflow artifacts to store built binaries and only upload to github release or push to registry on release event. This way we can include container building as part of pull request validation and make it easier to test changes.