Use full path to nsenter in docker-enter

I was trying to use docker-enter for the first time under boot2docker and got this error:

```
/var/lib/boot2docker/docker-enter: line 12: nsenter: not found
2014/07/15 09:13:20 exit status 127
```

I updated the docker-enter script to get the full path to nsenter based on the dirname of docker-enter itself.