Add Docker environment & web demo

[bfirsh]

Hey @kdexd! 👋

This pull request makes it possible to run your model inside a Docker environment, which makes it easier for other people to run it. We're using an open source tool called Cog to make this process easier.

This also means we can make a web page where other people can try out your model! We've implemented image captioning, but it should be pretty easy to add other tasks too if you'd like. View it here: https://replicate.ai/kdexd/virtex-image-captioning

That page also has instructions on how to use the Docker image, which is on our registry at r8.im/kdexd/virtex-image-captioning.

In case you're wondering who the heck I am, I'm from Replicate, where we're trying to make machine learning reproducible. So many cool models are being made, but I got frustrated that I couldn't run them, hence we're trying to fix that. :)