[FEAT] add python3, python3-pip, and python3-venv to image to allow use of most post proccesing scripts

[Nathan22211]

Is this a new feature request?

• I have searched the existing issues

Wanted change

add those 3 packages to the docker image and add a bind mount for post scripts to the compose file

another alteritive might be to bind mount the host's python install or a portable linux install to the container.

Reason for change

this is mostly because a vast majority of post proccesing scripts for Sli3er based slicers are written in python, currently they'd have to be compiled

Proposed code change

for the dockerfile, add apt-get install -y --no-install-recommends \ python3 python3-venv python3-pip

and for the compose file, add a bind mount for post scripts

[github-actions]

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[aptalca]

https://github.com/linuxserver/docker-mods/tree/universal-package-install