fix(stacks): avoid resource busy error in Docker fallback deletion (#271)

The previous command `rm -rf /cleanup` tried to remove the bind mount
point itself, which the kernel rejects with EBUSY. Changed to
`find /cleanup -mindepth 1 -maxdepth 1 -exec rm -rf {} +` which
removes all contents without touching the mount point. The existing
fsPromises.rmdir() call then cleans up the empty host directory.

backend/src/__tests__/filesystem.test.ts

@@ -93,7 +93,7 @@ describe('FileSystemService.deleteStack', () => {
     await expect(promise).resolves.toBeUndefined();
     expect(mockSpawn).toHaveBeenCalledWith(
       'docker',
-      expect.arrayContaining(['run', '--rm', '-v', expect.stringContaining(':/cleanup'), 'alpine', 'rm', '-rf', '/cleanup']),
+      expect.arrayContaining(['run', '--rm', '-v', expect.stringContaining(':/cleanup'), 'alpine', 'sh', '-c', 'find /cleanup -mindepth 1 -maxdepth 1 -exec rm -rf {} +']),
       expect.objectContaining({ env: expect.any(Object) }),
     );
   });