Support Docker integration on Windows

I'm trying to get any stack command to work with

``` yaml
docker:
enabled: true
```

After enabling docker, stack told me to use the comman `stack docker pull`, which can through fine and installed the docker image.

From now on, every time I try any stack command, stack quits right away with the message:

```
System.PosixCompat.User.getEffectiveUserID: not supported: illegal operation
```

I really want to use Docker here to get a consistent environment. Using stack: `Version 1.1.2, Git revision c6dac65e3174dea79df54ce6d56f3e98bc060ecc (3647 commits) x86_64 hpack-0.14.0` on newest Windows 10 Pro, and docker `version 1.12.0, build 8eab29e, experimental`.

_EDIT_: I'm pretty sure it is somehow related to [this line](https://github.com/commercialhaskell/stack/blob/e494b90fcb6ab0218af7ed6d4755e22a3f260f75/src/Stack/Docker.hs#L110), but I don't understand enough of it to try and fix it.