Use production build of React in deployment

[RichDom2185]

Not sure if this is a build configuration error or whether the message is safe to ignore as I did not have the time to investigate.

[kxrt]

We attempted this in #28, which switched the frontend Dockerfile to use Vite preview. However, as noted in sveltejs/kit#3004, Vite does not look at .env in a production build, so we'll need to rework our secrets for this.

Currently, we're copying over .env as .env.production into the frontend folder manually so that Vite can statically replace these into the built files.