Cannot build image

[ngiger]

Hi

I checked out your master tree (at 0aff1eed5588b3d14096e3c88930eeaefa1abd69)

Then I tried to follow the your readme. Vagrant up and vagrant ssh work fine. Same for nix-channel --update and nix-env -i hello

But trying to build an image failed with

 sudo -E /home/vagrant/nix-docker/nix-docker/bin/nix-docker  -t my-image nix-docker/samples/wordpress.nix 
Temp dir /tmp/tmp8Jqy2o
Building Nix closure for the image...
error: anonymous function at "/nix/store/34jk0lxmn8p8803yizqdh80a56kfhj3j-nixpkgs-15.05pre55774.b6f8d1f/nixpkgs/pkgs/servers/http/apache-httpd/2.4.nix":1:1 called with unexpected argument ‘mpm’, at "/nix/store/34jk0lxmn8p8803yizqdh80a56kfhj3j-nixpkgs-15.05pre55774.b6f8d1f/nixpkgs/lib/customisation.nix":58:12
(use ‘--show-trace’ to show detailed location information)
Traceback (most recent call last):
  File "/home/vagrant/nix-docker/nix-docker/bin/nix-docker", line 81, in <module>
    package_nix_path = build()
  File "/home/vagrant/nix-docker/nix-docker/bin/nix-docker", line 42, in build
    nix_path = subprocess.check_output(cmd)
  File "/usr/lib/python2.7/subprocess.py", line 573, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command '['nix-build', '/home/vagrant/nix-docker/nix-docker/docker.nix', '-I', 'configuration=nix-docker/samples/wordpress.nix', '--arg', 'mountBuild', 'true', '--argstr', 'name', 'my-image', '--argstr', 'baseImage', 'busybox']' returned non-zero exit status 1
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