fix: resolve PermissionError during local mode cleanup of root-owned Docker files

[mollyheamazon]

Issue

Local container training fails with PermissionError during post-training cleanup.
Docker containers write files as root, and shutil.rmtree cannot remove them when
running as a non-root user (e.g., sagemaker-user in Studio).

Root Cause

The original _rmtree attempted os.chmod on PermissionError, which also fails
on root-owned files when running as non-root.

Fix

On PermissionError, use docker run chmod -R 777 (as root inside the container)
to fix permissions, then retry shutil.rmtree. In SageMaker Studio, --network sagemaker is added since Studio restricts Docker to that network. The already-pulled
training image is reused to avoid pulling additional images.

Changes

• sagemaker-train/.../local/local_container.py — updated _rmtree, pass image
  and is_studio from train() cleanup calls
• sagemaker-core/.../local_core/local_container.py — same fix, kept in sync
• Added unit tests for _rmtree covering normal, fallback, Studio, and error cases

Testing

• Reproduced and verified fix on SageMaker Studio (Docker-enabled JupyterLab space)
• Unit tests pass for both sagemaker-train (5 new) and sagemaker-core (31 existing)

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