Use miniconda Python install in Docker images

[bandish-shah]

What does this PR do?

Previously we installed a new system Python and overrode the original. This caused issues with several packages such as PyPi and setuptools due to the venv scheme being patched into Python in the deadsnakes repo.

This PR switches the Python install to use miniconda. It also resolves the confusion between using site-packages and dist-packages (Debian).

What issue(s) does this change relate to?

TBD

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