containertool should warn if the user attempts to stack a mach-O executable on top of a Linux base image

When using `containertool` directly, or if accidentally using the plugin without a `--swift-sdk`, it is possible to end up layering a mach-O (macOS) executable on top of a Linux container image. This will not run, but the error messages which are printed when trying to run such an image can be difficult to understand.

`containertool` already tries to read an ELF header from the executable, in order to choose the correct base image architecture. It could also detect that the executable is a mach-O binary and warn the user.

Start by reading the containertool path that currently reads the executable's ELF header to choose the base-image architecture. Determine how it can identify a mach-O executable and where a warning should be emitted when it is layered onto a Linux container image; done means the invalid combination produces a clear warning.

Written by the indexing model from the issue text.