Investigate possible turning of `open_containerfile_directives()` into a context manager

[thrix]

Context

In tmt/steps/provision/__init__.py:

full_script = ShellScript.from_scripts(script_parts)

# Add to the package manager's engine
self.package_manager.engine.open_containerfile_directives()

The open_containerfile_directives() method should be refactored to be a context manager for better resource handling.

Original discussion

#4525 (comment)