Multi architecture builds

[tombeynon]

Currently Omnibus builds images for linux/amd64 exclusively. It would be helpful to have images for linux/arm/v7 and linux/arm64 to support all the standard architectures, but there are challenges with this:

1. Builds will take considerably longer. If we use platform emulation to build the other architectures then this will likely be ~5x longer, and builds already take ~20 minutes, so we'd be looking at 2 hours+ to build all images.
2. A number of the chains build images using pre-published binaries, so these would need to be adapted to use the correct binary for the given architecture.

If we do go down this path, we'd want to see if we could parallelise the multi-arch builds, avoid emulation when building non-amd64 images, and we'd likely want to remove a number of unused chains from the repository.