dockerTools.usrBinEnv: add

This provides a /usr/bin/env, for shell scripts using the
"/usr/bin/env executable" shebang.

pkgs/build-support/docker/default.nix

@@ -759,6 +759,13 @@ rec {
     ];
   };
 
+  # This provides a /usr/bin/env, for shell scripts using the
+  # "#!/usr/bin/env executable" shebang.
+  usrBinEnv = runCommand "usr-bin-env" { } ''
+    mkdir -p $out/usr/bin
+    ln -s ${pkgs.coreutils}/bin/env $out/usr/bin
+  '';
+
   # This provides /bin/sh, pointing to bashInteractive.
   binSh = runCommand "bin-sh" { } ''
     mkdir -p $out/bin