Use Python virtual environment

[gronke]

A previous Pull-Request #8 has added the --break-system-packages flag to the pip command. In response Python system packages, which are also installed via Docker, will break on update.

This PR installs the system package python-venv and configures a virtual environment in /opt/venv/.

[PeterMosmans]

What's the added value of using a virtual environment here for an immutable image (that gets hopefully built / updated quite often)?

[gronke]

What's the added value of using a virtual environment here for an immutable image (that gets hopefully built / updated quite often)?

Since Python PIP version 23.0.1 (2023-02-17) it requires the --break-system-packages flag when installing packages without virtual environment.

This repo mixes system packages (installed from Debian apt) and Python PIP, which may conflict. One obvious way to resolve this issue is adding the --break-system-packages, though it comes with a catch: It breaks when system packets are updated at a later time. And actually we do want to be able to do that.

This PR opts for the virtual environment instead, which fixes the issue in both cases.

[PeterMosmans]

Thanks for the explanation @gronke - however, what's against doing something like

RUN apt-get install -y python3 python3-pip python3-tz pandoc python3-pypandoc python3-gitlab python3-slugify

where all packages are system packages?

[gronke]

Would you please create a PR and close this one in case you prefer the other solution? One way or the other, builds are currently broken. I don't mind either solution.

[PeterMosmans]

It was just a question as I was curious for the reason of using venv instead of system packages - all good from my side this solution 👍