Fix UV cache permission error in sandbox container

[Copilot]

UV fails to initialize cache in the sandbox container due to permission denied on /home/sandboxuser/.cache/uv. The tmpfs mount at /home/sandboxuser/.cache (configured in compose.yml) inherits incorrect ownership when the directory doesn't exist in the image.

Changes

• Create /home/sandboxuser/.cache directory with proper ownership in sandbox/Dockerfile before switching to non-root user

# Before
RUN adduser -D sandboxuser
USER sandboxuser

# After
RUN adduser -D sandboxuser && \
    mkdir -p /home/sandboxuser/.cache && \
    chown -R sandboxuser:sandboxuser /home/sandboxuser/.cache
USER sandboxuser

The directory now exists with correct ownership when the tmpfs mount is applied at runtime, allowing UV to initialize its cache successfully.

Original prompt

I keep getting this error:

error: Failed to initialize cache at `/home/sandboxuser/.cache/uv`
  Caused by: failed to create directory `/home/sandboxuser/.cache/uv`: Permission denied (os error 13)

When I run docker logs --follow sandbox. Please fix.

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[Copilot]

Pull request overview

This PR fixes a permission error preventing UV from initializing its cache in the sandbox container. The issue occurs because the tmpfs mount at /home/sandboxuser/.cache (configured in compose.yml) inherits incorrect ownership when the directory doesn't exist in the image.

Changes:

• Creates /home/sandboxuser/.cache directory with proper ownership in the Dockerfile before switching to the non-root user
• Ensures the directory exists with correct permissions when the tmpfs mount is applied at runtime

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