build: add dockerImage dev env package to nix flake

[vindard]

Description

This adds a package to nix.flake that builds an image with the nativeBuildInputs as the environment.

Build with:

$ nix build .#dockerImage

Load with:

$ docker load < result

Run with:

$ docker run -it galoy-dev-env:latest

---

Nix Outputs

From running $ nix flake show:

git+file:///home/arvinda/Developer/Projects/GaloyMoney/galoy?ref=refs%2fheads%2fdocker-nix-image&rev=9808c4b59205a08786dba2fe44c9daf42ff2ee41
├───devShells
│   ├───aarch64-darwin
│   │   └───default: development environment 'nix-shell'
│   ├───aarch64-linux
│   │   └───default: development environment 'nix-shell'
│   ├───x86_64-darwin
│   │   └───default: development environment 'nix-shell'
│   └───x86_64-linux
│       └───default: development environment 'nix-shell'
├───formatter
│   ├───aarch64-darwin: package 'alejandra-3.0.0'
│   ├───aarch64-linux: package 'alejandra-3.0.0'
│   ├───x86_64-darwin: package 'alejandra-3.0.0'
│   └───x86_64-linux: package 'alejandra-3.0.0'
└───packages
    ├───aarch64-darwin
    │   ├───api: package 'api'
    │   ├───api-cron: package 'api-cron'
    │   ├───api-exporter: package 'api-exporter'
    │   ├───api-trigger: package 'api-trigger'
    │   ├───api-ws-server: package 'api-ws-server'
    │   ├───consent: package 'consent'
    │   ├───dashboard: package 'dashboard'
    │   └───dockerImage: package 'galoy-dev-env.tar.gz'
    ├───aarch64-linux
    │   ├───api: package 'api'
    │   ├───api-cron: package 'api-cron'
    │   ├───api-exporter: package 'api-exporter'
    │   ├───api-trigger: package 'api-trigger'
    │   ├───api-ws-server: package 'api-ws-server'
    │   ├───consent: package 'consent'
    │   ├───dashboard: package 'dashboard'
    │   └───dockerImage: package 'galoy-dev-env.tar.gz'
    ├───x86_64-darwin
    │   ├───api: package 'api'
    │   ├───api-cron: package 'api-cron'
    │   ├───api-exporter: package 'api-exporter'
    │   ├───api-trigger: package 'api-trigger'
    │   ├───api-ws-server: package 'api-ws-server'
    │   ├───consent: package 'consent'
    │   ├───dashboard: package 'dashboard'
    │   └───dockerImage: package 'galoy-dev-env.tar.gz'
    └───x86_64-linux
        ├───api: package 'api'
        ├───api-cron: package 'api-cron'
        ├───api-exporter: package 'api-exporter'
        ├───api-trigger: package 'api-trigger'
        ├───api-ws-server: package 'api-ws-server'
        ├───consent: package 'consent'
        ├───dashboard: package 'dashboard'
        └───dockerImage: package 'galoy-dev-env.tar.gz'