Allow building with a custom derivation

examples/custom-plan/flake.nix

@@ -0,0 +1,30 @@
+{
+  description = "A custom plan example for Packsnap";
+
+  inputs.packsnap.url = "../..";
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  outputs = { self, nixpkgs, flake-utils, packsnap }: flake-utils.lib.eachDefaultSystem (system:
+      let pkgs = (import nixpkgs) { inherit system; };
+      in {
+        packages.packsnap-custom-plan = packsnap.lib.${system}.buildCustomPlan { 
+          name = "packsnap-custom-plan";
+          contents = [
+            (pkgs.stdenv.mkDerivation {
+              name = "packsnap-custom-plan";
+              inherit system;
+
+              src = ./.;
+              nativeBuildInputs = [
+                pkgs.gcc
+                pkgs.pkg-config
+                pkgs.meson
+                pkgs.ninja
+              ];
+            })
+          ];
+          start = "packsnap-custom-plan";
+        };
+      }
+  );
+}

examples/custom-plan/main.c

@@ -0,0 +1,12 @@
+#include <stdio.h>
+
+#define PROJECT_NAME "packsnap-custom-plan"
+
+int main(int argc, char **argv) {
+    if(argc != 1) {
+        printf("%s takes no arguments.\n", argv[0]);
+        return 1;
+    }
+    puts("Hello, Packsnap from C!");
+    return 0;
+}

examples/custom-plan/meson.build

@@ -0,0 +1,8 @@
+project('custom-plan', 'c',
+  version : '0.1',
+  default_options : ['warning_level=3'])
+
+exe = executable('packsnap-custom-plan', 'main.c',
+  install : true)
+
+test('basic', exe)

lib/default.nix

@@ -3,10 +3,14 @@
 , misc ? import ./misc.nix {}
 , naersk
 , npmlock2nix }:
-let providers = import ./providers { inherit pkgs naersk npmlock2nix; };
+let 
+  providers = import ./providers { inherit pkgs naersk npmlock2nix; };
+  plan = import ./plan { inherit pkgs; };
 in {
-  build = ({ name ? "packsnap-image", path }:
+  build = ({ name ? "packsnap-image", tag ? "latest", path }:
     let provider = providers.planBuild path;
-    in pkgs.dockerTools.buildImage ((provider.plan path) // { name = name; tag = "latest"; })
+    in pkgs.dockerTools.buildImage ((provider.plan path) // { inherit name tag; })
   );
+  buildCustomPlan = ({ name ? "packsnap-image", tag ? "latest", variables ? [], contents, libraries ? [], start }:
+    pkgs.dockerTools.buildImage ((plan.buildPlan variables contents libraries start) // { inherit name tag; }));
 }