### What changes were proposed in this pull request?
Add cross build support to our docker image script using the new dockerx extension.
### Why are the changes needed?
We have a CI for Spark on ARM, we should support building images for ARM and AMD64.
### Does this PR introduce _any_ user-facing change?
Yes, a new flag is added to the docker image build script to cross-build
### How was this patch tested?
Manually ran build script & pushed to https://hub.docker.com/repository/registry-1.docker.io/holdenk/spark/tags?page=1 verified amd64 & arm64 listed.
Closes#28615 from holdenk/cross-build.
Lead-authored-by: Holden Karau <hkarau@apple.com>
Co-authored-by: Holden Karau <holden@pigscanfly.ca>
Signed-off-by: Holden Karau <hkarau@apple.com>
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