nix: init images

[SomeoneSerge]

Exposes a few attributes demonstrating how to build singularity/apptainer and docker images re-using llama.cpp's Nix expression

(cherry-picks from #4917, needs rebasing later)

[ggerganov]

@SomeoneSerge Is this PR still relevant, or we can close it?

[SomeoneSerge]

@ggerganov sorry I got distracted and forgot to finish this. I'd still like to merge this, I just need to allocate an evening for polishing

EDIT: I skimmed through the changes and I can't recall what was it I wanted to add before merging
EDIT2: I'll rebase and push-force to trigger the CI

[philiptaron]

Built locally on x86_64-linux with nix build github:someoneserge/llama.cpp/feat/nix/images#llamaPackages.{docker,docker-min,sif,llama-cpp} and it's fast and effective.

[philiptaron]

I intend to merge when CI completes. @ggerganov, perhaps you could turn on the ability to automerge after CI in the repo settings? Maybe the repo rules aren't set up to make that effective, though, since it says I can merge right now, so CI might not be a blocker.

[ggerganov]

Yes, merging can be done anytime if at least 1 approval is provided. But for now I'll keep the auto-merge disabled