Follow-up from PR #211.

It seems like gcloud supports python/flask deployments without docker. If this works, there may be several advantages:

• One less file for us to maintain, and we wouldn't need to worry about building and pushing docker images.
• Cold start-up may be quicker, since the python code is much smaller than the docker image.
• Performance may be better, because gcloud would be running it as a wsgi production server, rather than the dev server that the docker image currently provides.

Proposed path:

• introduce a requirements.in file and compile requirements.txt from that.
• add a new workflow for a new docker-less deployment
• Make a PR with just that much, and @FedericoTartarini can test
• if it works, then make a follow-up PR that removes the Dockerfile and updates the docs.

I'll try to looks at this on Friday, December 1.