Bump pillow from 6.2.1 to 8.1.1 in /docker/gpu/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/gpu/onnx/requirements.txt

@@ -8,7 +8,7 @@ flask-restplus==0.12.1
 sphinx-rtd-theme==0.4.3
 onnx==1.5.0
 onnxruntime-gpu==0.4.0
-Pillow==6.2.1
+Pillow==8.1.1
 pymediainfo==4.0
 boto3==1.9.199
 pytest==5.1.3