Add python-is-python3 package for bullseye base docker (#11895)

Why I did it
In latest syncd container, it is installed bullseye, can't find command '/usr/bin/python'.
Some scripts such as test_copp still calls /usr/bin/python in syncd.
Submitted the change in #11807 for syncd docker, but it's better to add it in bullseye base docker.

How I did it
Install python-is-python3 package in bullseye base docker to resolve this issue, whatever run python or python3, it will run /usr/bin/python3, will not cause the error of can't find command '/usr/bin/python'

How to verify it
run python in syncd container.

Signed-off-by: Zhaohui Sun <zhaohuisun@microsoft.com>

dockers/docker-base-bullseye/Dockerfile.j2

@@ -48,6 +48,7 @@ RUN apt-get update &&        \
         python3              \
         python3-distutils    \
         python3-pip          \
+        python-is-python3    \
         vim-tiny             \
 # Install redis-tools
         redis-tools          \