WIP: move more shared code into scripts

[jameslamb]

Closes #832

Follow-up to https://github.com/rapidsai/docker/pull/836#discussion_r2674112913

In #836, this repo's dependency on https://github.com/rapidsai/ci-imgs was removed by replacing FROM rapidsai/miniforge-cuda with the inline contents of the Dockerfile used to produce that image. That introduced a lot of duplication between the Dockerfiles there.

This proposes reducing much of that duplication by moving shared code into scripts that are mounted in at build time.