Bump pillow from 6.2.1 to 8.1.1 in /docker/cpu/onnx

Bumps [pillow](https://github.com/python-pillow/Pillow) from 6.2.1 to 8.1.1.
- [Release notes](https://github.com/python-pillow/Pillow/releases)
- [Changelog](https://github.com/python-pillow/Pillow/blob/master/CHANGES.rst)
- [Commits](python-pillow/Pillow@6.2.1...8.1.1)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docker/cpu/onnx/requirements.txt

@@ -8,7 +8,7 @@ flask-restplus==0.12.1
 sphinx-rtd-theme==0.4.3
 onnx==1.9.0
 onnxruntime==1.5.1
-Pillow==6.2.1
+Pillow==8.1.1
 celery==4.3.0
 redis==3.3.6
 vine==1.3.0