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/distribution/build.py

@@ -93,6 +93,7 @@ def build_image(
     image_name: str,
     template_or_config: str,
     run_config: str | None = None,
+    export_dir: str | None = None,
 ):
     container_base = build_config.distribution_spec.container_image or "python:3.10-slim"
 
@@ -108,11 +109,18 @@ def build_image(
             container_base,
             " ".join(normal_deps),
         ]
+        if export_dir is not None:
+            args.append("--export-dir")
+            args.append(f"{export_dir}")
 
         # When building from a config file (not a template), include the run config path in the
         # build arguments
         if run_config is not None:
             args.append(run_config)
+
+        if special_deps:
+            args.append("--special-pip-deps")
+            # The content is added after all the image_type conditions
     elif build_config.image_type == LlamaStackImageType.CONDA.value:
         script = str(importlib.resources.files("llama_stack") / "distribution/build_conda_env.sh")
         args = [