chore: Upgrade to python:3.9-slim-bullseye to fix docker build

Author: jakub-bochenski

docker/Dockerfile

@@ -1,7 +1,7 @@
 # Copyright (c) 2019-2021 VMware, Inc. All Rights Reserved.
 # SPDX-License-Identifier: BSD-2-Clause
 
-FROM python:3.9-slim-buster as base
+FROM python:3.9-slim-bullseye as base
 
 FROM base as builder
 
@@ -13,9 +13,7 @@ RUN pip install --no-warn-script-location --prefix=/install \
 
 FROM base
 
-RUN echo "deb http://deb.debian.org/debian bullseye main" > /etc/apt/sources.list.d/bullseye.list \
-    && echo "Package: *\nPin: release n=bullseye\nPin-Priority: 50" > /etc/apt/preferences.d/bullseye \
-    && apt-get update \
+RUN apt-get update \
     && apt-get install -y --no-install-recommends \
     attr \
     findutils \