fix(cli): support context-less builds via standard input (fixes #2139)

[manideep-malyala]

Description

This PR addresses issue #2139 by adding support for context-less builds via container build - < Dockerfile, aligning with standard Docker CLI behavior.

Changes Made

• Validation Bypass: Updated BuildCommand.swift's validate() method so that if contextDir is "-", it correctly bypasses the FileManager.default.fileExists check (which previously threw context dir does not exist -).
• Dockerfile Defaulting: If no -f is explicitly provided and contextDir is "-", dockerfile correctly defaults to "-" (stdin).
• Empty Context Initialization: Modified the run() closure to generate and pass an empty temporary directory (empty-context) to the Builder.BuildConfig when contextDir is "-". This satisfies the underlying ContainerBuild engine requirements while gracefully supplying a truly empty context to the builder.

Fixes #2139.