Wrap arion binary

PATH+=docker-compose, unset PYTHONPATH

nix/overlay.nix

@@ -1,6 +1,7 @@
 self: super:
 let
   inherit (self.arion-project) haskellPkgs;
+  inherit (super) lib;
 
   srcDir = ../src; # TODO gitignoreSource + whitelist nix and arion-image
   eval = import (srcDir + "/nix/eval-composition.nix");
@@ -13,10 +14,30 @@ in
 
   arion-v0 = super.callPackage ../arion.nix {};
   arion = hlib.justStaticExecutables (hlib.overrideCabal haskellPkgs.arion-compose (o: {
+      buildTools = o.buildTools ++ [super.makeWrapper];
       passthru = o.passthru // {
         inherit eval build;
       };
       pname = "arion"; # Cover up the needlessly long Haskell package name
+
+      # PYTHONPATH
+      #
+      # We close off the python module search path!
+      #
+      # Accepting directories from the environment into the search path
+      # tends to break things. Docker Compose does not have a plugin
+      # system as far as I can tell, so I don't expect this to break a
+      # feature, but rather to make the program more robustly self-
+      # contained.
+
+      postInstall = ''${o.postInstall or ""}
+        mkdir -p $out/libexec
+        mv $out/bin/arion $out/libexec
+        makeWrapper $out/libexec/arion $out/bin/arion \
+          --unset PYTHONPATH \
+          --prefix PATH : ${lib.makeBinPath [ self.docker-compose ]} \
+          ;
+      '';
     }));
   tests = super.callPackage ../tests {};
   doc = super.callPackage ../doc {};

tests/arion-test/default.nix

@@ -12,7 +12,6 @@ in
   machine = { pkgs, lib, ... }: {
     environment.systemPackages = [
       pkgs.arion
-      pkgs.docker-compose
     ];
     virtualisation.docker.enable = true;