Dockerize Webapp

[beekley]

addresses #326 (semi-related)

Motivation and context

To run end-to-end tests, we'll have to have a real instance of the app running. Docker will help simplify this process by handling the setup/build/running of the app. It will also make the development environments more consistent since everyone will run the same environment in a container.

What I did

• Update the Dockerfile for the node webapp, which builds the dependencies in the container while keeping the code local to the host. This allows nodemon to continue to work.
• Reorganize the config files to support an additional dev environment (dev-docker).
• Update the readme to use docker. This could like be simplified further by moving the frontend/mongo setup steps to the Dockerfile.

I'd like a reviewer to test running docker on their machine.

[beekley]

Looks redundant w/ https://github.com/helloGov/webapp/pull/334

[beekley]

Built on #334