feat: --image-type argument overrides value in --config build.yaml (#2179)

closes #2162

# test plan

run `llama stack build --image-name ollama --image-type
<venv/conda/container> --config llama_stack/templates/ollama/build.yaml`
and verify venv | conda | container are built.

llama_stack/cli/stack/_build.py

@@ -206,6 +206,8 @@ def run_stack_build_command(args: argparse.Namespace) -> None:
                 contents = yaml.safe_load(f)
                 contents = replace_env_vars(contents)
                 build_config = BuildConfig(**contents)
+                if args.image_type:
+                    build_config.image_type = args.image_type
             except Exception as e:
                 cprint(
                     f"Could not parse config file {args.config}: {e}",

llama_stack/cli/stack/build.py

@@ -49,7 +49,7 @@ class StackBuild(Subcommand):
             type=str,
             help="Image Type to use for the build. If not specified, will use the image type from the template config.",
             choices=[e.value for e in ImageType],
-            default=ImageType.CONDA.value,
+            default=None,  # no default so we can detect if a user specified --image-type and override image_type in the config
         )
 
         self.parser.add_argument(