$PWD is the -w workdir for docker builds but $PWD isn't mounted

The code changes the docker -w workdir to $PWD but then doesn't mount $PWD. The code assumes (the most common case) that $PWD is under your project directory.

```
mkdir /tmp/pants
cd /tmp/pants
stack --stack-yaml=$HOME/src/bannanas/stack.yaml exec -- bash
```

_crashing locomotive sound_