Feature request

Is your feature request related to a problem? Please describe.

Currently, built-in environments allows only .env.development and .env.production environments, so when you run yarn dev it uses .env.development or .env.local and when you do yarn build, then yarn start it uses .env.production environment. But there's no way to use a staging environment for example, like .env.staging, there's no way to run app with staging environment, if we try to NODE_ENV=staging yarn build && yarn start we get an error: "https://github.com/zeit/next.js/blob/master/errors/non-standard-node-env.md", and it won't have effect, because NODE_ENV would be production anyway, it's being overwritten.

Describe the solution you'd like

There should be a way to set environment when you run your app, like NODE_ENV=staging/prestaging yarn build && yarn start, etc. In CRA i use it this way:

"build:staging": "env-cmd -f .env.staging yarn build",
"build:prestaging": "env-cmd -f .env.prestaging yarn build"

Would be nice to have the same, but built-in in Next.

PS

"build:staging": "env-cmd -f .env.staging yarn build && yarn start" this actually works with Next either, though NODE_ENV would be set to "production".