`containertool` should build multi-arch images

`containertool` currently builds single-architecture container images (the top object is an architecture-specific `Manifest`). It should build multi-arch images (the top object is an `ImageIndex` which points to several architecture-specific `Manifest` objects).

Currently, a SwiftPM plugin cannot build two binaries in one invocation, however `containertool` could build a multi-arch binary if several architecture-specific binaries were passed in. ELF type detection (#49) would make this quite straightforward: `containertool` could accept several binaries for different architectures as command line arguments, detect the architecture of each binary and build an architecture-specific `Manifest`, then build an `ImageIndex` linking them all together.

Start by tracing containertool's current single-architecture image build and review the ELF type detection work in issue #49. The issue describes accepting multiple architecture-specific binaries, creating a Manifest for each, and producing an ImageIndex linking them; done means the tool can build a multi-arch image from those inputs.

Written by the indexing model from the issue text.