[buildah] support pre-script/post-script & manifest push

[terryzwt]

Expected Behavior

In my case, I want to build multiarch image using below flow:
a. buildah manifest rm buildah-test1
b. buildah build --platform linux/amd64,linux/arm64 --manifest buildah-test1 -t docker.io/example/buildah-test .
c. buildah manifest push --all buildah-test1 docker://docker.io/example/buildah-test

Actual Behavior

1. Can not meet requirement of step a.
2. Can not meet requirement of step c.

[terryzwt]

Another idea is that buildah task support multiarch image build native:)