Update python38-core-tools.Dockerfile

[TsuyoshiUshio]

Azure CLI doesn't support bullseye for now. However, python image looks upgraded to the bullseye.
I fix the version for time being.

Currently this issue, block releasing Azure Functions Core Tools image.

Context

We found the Core Tools Publish pipeline is fail by the following error. The pipeline is for releasing Core Tools docker images that runs nightly. The image is used mainly for devcontainer.
https://azure-functions.visualstudio.com/azure-functions-docker/_build/results?buildId=11733&view=results

That caused by the update of the OS on the python official image.

#5 3.229 E: The repository 'https://packages.microsoft.com/repos/azure-cli bullseye Release' does not have a Release file.

We found that python official images has move from buster to bullseye.
docker-library/python#633

Found mcr.microsoft.com/vscode/devcontainers/python:0.201.8-3.8 is still on the buster.
We are rollback the change or use new ways once Azure CLI team release the Bullseye.

Azure/azure-cli#19370

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[vrdmr]

Thanks @TsuyoshiUshio for doing this.
LGTM

[pragnagopa]

@TsuyoshiUshio - Thanks for the PR! Can you please update the description with details on how this issue was discovered? Is releasing core tools images part of regular host releases?

[TsuyoshiUshio]

@pragnagopa I added the context, looks good? If not, let's have offline quick call.