Make it possible to choose platform

[Emilgardis]

cross/src/docker/image.rs

Lines 49 to 83 in 19be834

	} else {
	let same_arch = self
	.toolchain
	.iter()
	.filter(|platform| {
	&platform.architecture
	== engine.arch.as_ref().unwrap_or(&Architecture::Amd64)
	})
	.collect::<Vec<_>>();
	if same_arch.len() == 1 {
	// pick the platform with the same architecture
	same_arch.first().expect("should contain one element")
	} else if let Some(platform) = same_arch
	.iter()
	.find(|platform| &platform.os == engine.os.as_ref().unwrap_or(&Os::Linux))
	{
	*platform
	} else if let Some(platform) =
	same_arch.iter().find(|platform| platform.os == Os::Linux)
	{
	// container engine should be fine with linux
	platform
	} else {
	let platform = self
	.toolchain
	.first()
	.expect("should be at least one platform");
	// FIXME: Don't throw away
	msg_info.warn(
	format_args!("could not determine what toolchain to use for image, defaulting to `{}`", platform.target),
	).ok();
	platform
	}
	};

We should allow users to pick exactly which platform to run the docker image on.

related to #1498