requirements.txt doesn't exist, and is used in dockerfile.

Hi all.

The [dockerfile](https://github.com/junyanz/pytorch-CycleGAN-and-pix2pix/blob/master/docs/Dockerfile) for the project has this line:

```RUN mkdir /workspace/ && cd /workspace/ && git clone https://github.com/junyanz/pytorch-CycleGAN-and-pix2pix.git && cd pytorch-CycleGAN-and-pix2pix && pip install -r requirements.txt```

for a requirements.txt that doesn't exist.

Thank you.