Add lib and pkgs to repl

Side effect: expose them in eval-composition.

src/nix/eval-composition.nix

@@ -35,4 +35,8 @@ let
 
 in
   # Typically you need composition.config.build.dockerComposeYaml
-  composition
+  composition // {
+    # throw in lib and pkgs for repl convenience
+    inherit lib;
+    inherit (composition.config._module.args) pkgs;
+  }