Add arion.build, arion.eval passthru functions

arion.nix

@@ -1,8 +1,9 @@
 { stdenv, lib
 , coreutils, docker_compose, jq
 }:
+let
 
-stdenv.mkDerivation {
+  arion = stdenv.mkDerivation {
     name = "arion";
     src = ./src;
     unpackPhase = "";
@@ -18,4 +19,20 @@ stdenv.mkDerivation {
         ;
       chmod a+x $out/bin/arion
     '';
-}
+    inherit passthru;
+  };
+
+  passthru = {
+    inherit eval build;
+  };
+
+  eval = import "${nix_dir}/eval-composition.nix";
+
+  build = args@{...}:
+    let composition = eval args;
+    in composition.config.build.dockerComposeYaml;
+
+  nix_dir = "${arion.outPath}/share/arion/nix";
+
+in
+  arion

doc/manual/installation.asciidoc

@@ -17,6 +17,8 @@ nix-env -iA arion -f .
 TODO: describe: using nix-shell or in a script, building images as
       part of nix-build, pinning, see also todomvc-nix.
 
+TODO: exposed Nix functions: arion.build, arion.eval (a bit of IFD)
+
 == Mac or traditional Linux
 
 ```

tests/default.nix

@@ -1,8 +1,20 @@
 { pkgs ? import ../pkgs.nix }:
 let
-  inherit (pkgs) nixosTest recurseIntoAttrs;
+  inherit (pkgs) nixosTest recurseIntoAttrs arion;
 in
 
 recurseIntoAttrs {
+
   test = nixosTest ./arion-test;
+
+  testBuild = arion.build {
+
+    # To be more accurately, you can do
+    #   pkgs = import ../examples/minimal/arion-pkgs.nix;
+    # but this is quite efficient:
+    inherit pkgs;
+
+    modules = [ ../examples/minimal/arion-compose.nix ];
+  };
+
 }