Update docker image for async refactor, use new ACR

[sarah-widder]

Motivation and Context

#460 refactored the backend to use ASGI, while the docker container was still using WSGI. This caused new images after that change to fail to start with the old dockerfile.

Description

From @iseabock

• Update WebApp.Dockerfile to use gunicorn CMD
• Update requirements.txt to include gunicorn

From @sarah-widder

• Update workflow to push to a new ACR, to avoid unintentionally breaking apps using the pre-refactor container.
• Update 'Deploy to Azure' ARM template to point to the new ACR.

Contribution Checklist

• I have built and tested the code locally and in a deployed app
• This is a change for all users of this app. No code or asset is specific to my use case or my organization.
• I didn't break any existing functionality 😄

[iseabock]

Looks great!