Add Docker environment using Cog

[bfirsh]

This is a start at getting Pixray running on Replicate. You can now do this to run it in a consistent Docker environment:

cog run python pixray.py --drawer=pixel --prompt=sunrise --output myfile.png

I have also rewritten requirements.txt because it looked like they were not requirements for Pixray, but maybe there is stuff in there that was important.

[rvorias]

Awesome feature! Thanks!