build: update env variables according to nextjs docs

[dolcalmi]

Move all env variables to .env according to https://nextjs.org/docs/basic-features/environment-variables

Note: .env, .env.development, and .env.production files should be included in your repository as they define defaults. .env*.local should be added to .gitignore, as those files are intended to be ignored. .env.local is where secrets can be stored.