Update website Dockerfile to produce a production-ready image

[natearn]

The primary role of these Dockerfiles is to build images for production. Ideally, these images can also be used as-is for development, but I don't expect that to necessarily be satisfiable.

Improvements:

• Replace the server with something that claims to be intended for running in a production environment.
• Run the server as an unprivileged user (not root).
• Separate dependencies from source code to speed up builds.

Considerations:

• A server that can be configured to use hot-reloading on source file changes.
• A server that can be configured to produce logs that are valuable for development and debugging.

[natearn]

The current website image is actually very frustrating to work with in dev also. I'm going to make this a priority.