Support multiple versions of python in parallel

[Aelire]

The idea is to use Docker multi-stage builds to bring python from multiple images into a single image

The base image (the first FROM at the top of the Dockerfile) will bring in the /usr/local/bin/python3 symlink, so for things running outside a venv like the rando bot itself, that will be the version used
The other versions only have a /usr/local/bin/python<version> executable and need to be called with a full version name. Here that's only 3.10 but we could include any other version for which there is an official docker image with the same construct in the future. The process is somewhat manual/copy-pasty but shouldn't be too difficult to maintain, since python installations are fairly self-contained with a binary, a shared library and everything else in the /usr/lib/python<version> directory (this has been true since at least python2 as far as I can tell)

I've tested this under python 3.12 for randobot and wwrando-random-settings with the gclib/python3.12 patches removed in this test race: https://racetime.gg/twwr/hyper-molgera-4964

[wooferzfg]

Prior to merging this, I'll push a change to update the mixed pools build branch with the same change