fix: use Unraid docker GID (281) instead of standard (999)

Fixes Docker socket permission denied error when running on Unraid.
The container's docker group must match the host's docker GID for
socket access to work properly.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Dockerfile

@@ -84,7 +84,8 @@ RUN curl -fsSL https://download.docker.com/linux/debian/gpg | gpg --dearmor -o /
     && rm -rf /var/lib/apt/lists/*
 
 # Create docker group first, then create a non-root user for running the application
-RUN groupadd -g 999 docker 2>/dev/null || true \
+# Note: GID 281 matches Unraid's docker group for socket access
+RUN groupadd -g 281 docker 2>/dev/null || true \
     && useradd -m -u 1001 -s /bin/bash claudeuser \
     && usermod -aG docker claudeuser 2>/dev/null || true