add default build.options to specify platform for image & feature building

[DanBradbury]

Proposed fix for #76

Not sure of a cleaner way to fix the arch building differences when run_args will not be sufficient in applying the setting to the features

{
	"image": "ghcr.io/rails/devcontainer/images/ruby:3.3.0",
	"runArgs": ["--platform=linux/amd64"],
...
}

[rafaelfranca]

We should not only build those feature in the the amd64 platform. We build the images in the platforms linux/amd64 and linux/arm64.