feat: allow building distro with external providers (#1967)

# What does this PR do?

We can now build a distribution that includes external providers.
Closes: https://github.com/meta-llama/llama-stack/issues/1948

## Test Plan

Build a distro with an external provider following the doc instructions.

[//]: # (## Documentation)

Added.

Rendered:


![Screenshot 2025-04-18 at 11 26
39](https://github.com/user-attachments/assets/afcf3d50-8d30-48c3-8d24-06a4b3662881)

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

llama_stack/cli/stack/_build.py

@@ -210,16 +210,9 @@ def run_stack_build_command(args: argparse.Namespace) -> None:
                 )
                 sys.exit(1)
 
-        if build_config.image_type == LlamaStackImageType.CONTAINER.value and not args.image_name:
-            cprint(
-                "Please specify --image-name when building a container from a config file",
-                color="red",
-            )
-            sys.exit(1)
-
     if args.print_deps_only:
         print(f"# Dependencies for {args.template or args.config or image_name}")
-        normal_deps, special_deps = get_provider_dependencies(build_config.distribution_spec.providers)
+        normal_deps, special_deps = get_provider_dependencies(build_config)
         normal_deps += SERVER_DEPENDENCIES
         print(f"uv pip install {' '.join(normal_deps)}")
         for special_dep in special_deps:
@@ -274,9 +267,10 @@ def _generate_run_config(
         image_name=image_name,
         apis=apis,
         providers={},
+        external_providers_dir=build_config.external_providers_dir if build_config.external_providers_dir else None,
     )
     # build providers dict
-    provider_registry = get_provider_registry()
+    provider_registry = get_provider_registry(build_config)
     for api in apis:
         run_config.providers[api] = []
         provider_types = build_config.distribution_spec.providers[api]
@@ -290,8 +284,22 @@ def _generate_run_config(
             if p.deprecation_error:
                 raise InvalidProviderError(p.deprecation_error)
 
-            config_type = instantiate_class_type(provider_registry[Api(api)][provider_type].config_class)
-            if hasattr(config_type, "sample_run_config"):
+            try:
+                config_type = instantiate_class_type(provider_registry[Api(api)][provider_type].config_class)
+            except ModuleNotFoundError:
+                # HACK ALERT:
+                # This code executes after building is done, the import cannot work since the
+                # package is either available in the venv or container - not available on the host.
+                # TODO: use a "is_external" flag in ProviderSpec to check if the provider is
+                # external
+                cprint(
+                    f"Failed to import provider {provider_type} for API {api} - assuming it's external, skipping",
+                    color="yellow",
+                )
+                # Set config_type to None to avoid UnboundLocalError
+                config_type = None
+
+            if config_type is not None and hasattr(config_type, "sample_run_config"):
                 config = config_type.sample_run_config(__distro_dir__=f"~/.llama/distributions/{image_name}")
             else:
                 config = {}
@@ -323,6 +331,7 @@ def _run_stack_build_command_from_build_config(
     template_name: Optional[str] = None,
     config_path: Optional[str] = None,
 ) -> str:
+    image_name = image_name or build_config.image_name
     if build_config.image_type == LlamaStackImageType.CONTAINER.value:
         if template_name:
             image_name = f"distribution-{template_name}"