feat: export distribution container build artifacts

Add a new --export-dir flag to the `llama stack build` command that
allows users to export container build artifacts to a specified
directory instead of building the container directly. This feature is
useful for:

- Building containers in different environments
- Sharing build configurations
- Customizing the build process

The exported tarball includes:
- Containerfile (Dockerfile)
- Run configuration file (if building from config)
- External provider files (if specified)
- Build script for assistance

The tarball is named with a timestamp for uniqueness: <distro-name>_<timestamp>.tar.gz

Documentation has been updated in building_distro.md to reflect this new
functionality as well as integration tests.

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/cli/stack/build.py

@@ -5,6 +5,7 @@
 # the root directory of this source tree.
 import argparse
 import textwrap
+from pathlib import Path
 
 from llama_stack.cli.stack.utils import ImageType
 from llama_stack.cli.subcommand import Subcommand
@@ -82,6 +83,13 @@ the build. If not specified, currently active environment will be used if found.
             help="Build a config for a list of providers and only those providers. This list is formatted like: api1=provider1,api2=provider2. Where there can be multiple providers per API.",
         )
 
+        self.parser.add_argument(
+            "--export-dir",
+            type=Path,
+            default=None,
+            help="Export the build artifacts to a specified directory instead of building the container. This will create a directory containing the Dockerfile and all necessary files to build the container.",
+        )
+
     def _run_stack_build_command(self, args: argparse.Namespace) -> None:
         # always keep implementation completely silo-ed away from CLI so CLI
         # can be fast to load and reduces dependencies