`naersk` + `rust-overlay` -> `rustPlatform`, use nix to build docker images, add CI

[getchoo]

Description of changes

• naersk & rust-overlay have been replaced with nixpkgs' rustPlatform
  • There isn't much benefit to using it in this case besides not having to specify a Cargo.lock. I think it's worth it to do that for less inputs, though
  • I also refactored the derivation to use callPackage, which gives users an easier time in overriding and helps keep the flake itself small
• Nix is used to build Docker images directly
  • Nix was already being used in the Dockerfile's build, but now everything can be done with just Nix
  • This should also result in a much smaller image, as only /bin/sh and warden's closure are included
• CI has been added
  • This builds warden itself, the aforementioned Docker image, and also helps keep the flake.lock up to date - similar to Dependabot

Relevant Issues

n/a

CC Maintainers

@IogaMaster

[IogaMaster]

Thanks, next steps are making it a bot that scans the nixpkgs pr's as well as a cli.