Fix: Support x86_64 architecture alias to prevent silent pull failure…

[Ronitsabhaya75]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

closes #1035

Users attempting to pull cross-architecture images with --arch x86_64 or --platform linux/x86_64 experience silent failures with no output or error messages.

The fix: Add architecture name normalization to accept x86_64 and x86-64 as aliases for amd64:

public init?(rawValue: String) {
    switch rawValue.lowercased() {
    case "arm64":
        self = .arm64
    case "amd64", "x86_64", "x86-64":
        self = .amd64
    default:
        return nil
    }
}

This enables commands like:

container image pull --arch x86_64 ubuntu:24.04
container image pull --platform linux/x86_64 alpine:latest

Testing Added ArchTests.swift with comprehensive test coverage for architecture name normalization and alias support.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

[jglogan]

containerd code for comparison: https://github.com/containerd/platforms/blob/main/database.go#L75

[Ronitsabhaya75]

@jglogan Thanks for the reference! I followed the same normalization approach as containerd's normalizeArch function.

This should make cross-architecture workflows consistent with Docker/containerd expectations.