diff --git a/Development.md b/Development.md
index 0231a7eafc..a51639aaa6 100644
--- a/Development.md
+++ b/Development.md
@@ -98,6 +98,11 @@ Please refer to [this README](./tests/integration/README.md) for details.
 1. Add your dependency in `pyproject.toml` or use `poetry add xxx`
 2. Update the poetry.lock file via `poetry lock --no-update`
 
+### 9. Use existing Docker image
+To reduce build time (e.g., if no changes were made to the client-runtime component), you can use an existing Docker container image. Follow these steps:
+1. Set the SANDBOX_RUNTIME_CONTAINER_IMAGE environment variable to the desired Docker image.
+2. Example: export SANDBOX_RUNTIME_CONTAINER_IMAGE=ghcr.io/all-hands-ai/runtime:0.9-nikolaik
+
 ## Develop inside Docker container
 
 TL;DR
diff --git a/tests/unit/test_agent_skill.py b/tests/unit/test_agent_skill.py
index 1f1bf720ad..ae295b0ddf 100644
--- a/tests/unit/test_agent_skill.py
+++ b/tests/unit/test_agent_skill.py
@@ -67,12 +67,6 @@ def _calculate_window_bounds(current_line, total_lines, window_size):
     return start, end
 
 
-def _generate_ruby_test_file_with_lines(temp_path, num_lines) -> str:
-    file_path = temp_path / 'test_file.rb'
-    file_path.write_text('\n' * num_lines)
-    return file_path
-
-
 def _capture_file_operation_error(operation, expected_error_msg):
     with io.StringIO() as buf:
         with contextlib.redirect_stdout(buf):