build: add flake (#102)

flake.nix

@@ -0,0 +1,45 @@
+{
+  # Adapted from: https://github.com/the-nix-way/dev-templates/blob/main/kotlin/flake.nix
+  description = "A Nix-flake-based Kotlin development environment";
+
+  inputs = { nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11"; };
+
+  outputs = { self, nixpkgs }:
+    let
+      javaVersion = 20;
+      # Some overlay to inject your dependencies (pkgs.jdk, pkgs.gradle, pkgs.kotlin) should be defined
+      overlays = [
+        (final: prev: rec {
+          jdk = prev."jdk${toString javaVersion}";
+          gradle = prev.gradle.override { java = jdk; };
+          kotlin = prev.kotlin.override { jre = jdk; };
+        })
+      ];
+      supportedSystems =
+        [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forEachSupportedSystem = f:
+        nixpkgs.lib.genAttrs supportedSystems
+        (system: f { pkgs = import nixpkgs { inherit system; }; });
+    in {
+      devShells = forEachSupportedSystem ({ pkgs }: {
+        # https://ryantm.github.io/nixpkgs/builders/special/fhs-environments/#sec-fhs-environments
+        # WARNING! As I understand, with this configuration your shell will be running in a namespace.
+        # It isolates the filesystem (or at least a part of the filesystem) to behave as a standard
+        # FHS system. gradlew should work normally, thus handling all your dependencies by itself.
+        # With this configuration, your only required dependency is the jdk17.
+        # Thus it is not a packaging flake, but rather a flake to enable development for your tool.
+        default = (pkgs.buildFHSUserEnv {
+          name = "java-dev-env";
+          targetPkgs = pkgs:
+            with pkgs; [
+              jdk17
+              # Gradle for building containers
+              gradle
+              # For the cli
+              docker-compose
+            ];
+          runScript = "bash";
+        }).env;
+      });
+    };
+}