Deduplicate server logic in vectara-answer

[cjcenizal]

We can update package.json scripts with this:

"serve": "node docker/server/index.js",

This will enable us to remove the server/ directory. We'll just need to update the serve script to also provide a command-line argument and then update docker/server/index.js to use that argument to conditionally set the port, the location of the build directory, and maybe some other things.