fix(docker): allow PYTHON_VERSION deviating from debian dist version

[3nprob]

Pull Request Checklist

Debian bullseye Python version (3.11) overrides build-arg PYTHON_VERSION due to entirety of /usr/local/bin being copied from runtime-deps, thereby removing the base image python installation.

This fixes it by changing the intermediary image to be based on one with the expected PYTHON_VERSION.

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[sandhose]

Debian bullseye Python version (3.11) overrides build-arg PYTHON_VERSION due to entirety of /usr/local/bin being copied from runtime-deps, thereby removing the base image python installation.

I'm confused by this statement, the purpose of this stage is only to gather extracted debs into /install-$ARCH, which is then copied into the actual runtime stage

Looking at the Python version in the end image confirms that it has the right version in it:

> docker run --rm --entrypoint='' ghcr.io/element-hq/synapse python --version
Python 3.12.9