use bash arrays for package lists, other misc. maintenance

[jameslamb]

Miscellaneous small cleanup and follow-ups.

Mount GH_TOKEN as environment variable secret

Follow-up from #336 (review)

Mounts the GitHub token used for gh cli calls as an environment variable secret, so commands don't need to cat it from the filesystem.

Adds floors on all conda and pip packages

I've added floors on every {conda,mamba,pip} install package in Dockerfiles here. Set them to the versions we're currently pulling in, which in all cases looked to me like the latest versions.

Benefits of doing this:

• faster solves
• reduced risk of silently pulling in very old versions of packages (which could result in hard-to-debug issues in CI)
• makes the impact of changes in the repo clearer (example: remove unnecessary gha-tools usage, put floors on conda dependencies docker#837 (comment))

Switches package lists to bash arrays

Follow-up from #342 (comment)

This makes it easier to change the set of installed packages based on details like CUDA version, OS, etc. (example: #199)

Adds a .gitignore

This project should have a .gitignore to avoid checking in unwanted files. I've proposed one here that I use in other projects.

Docs updates

• renamed section about building locally (you build "images" not "dockerfiles")
• documents gha-tools requirement in docs on building locally (current instructions fail with rapids-version-major-minor: command not found)
• changes formatting per https://yihui.org/en/2021/06/markdown-breath/

[gforsyth]

Yep, these all look like solid changes and the bash arrays will make additions less error-prone in the future.

[jameslamb]

Thanks for the review!