Vite-like envDir config

[eric-burel]

Goals

1. Loading .env from the root of a monorepo

Non-Goals

Background

We use a monorepo for the State of JavaScript surveys, running an API, 2 Next.js apps, an Astro app and a Gastby app.
They consume similar databases and thus need similar configuration. Handling .env files in each app is very confusing and requires a lot of management.

We'd be eager to load .env files from the root folder just above our Next app folder.

| surveyadmin
| surveyform
| .env # both apps load this .env

Calling loadEnvConfig from instrumentation.ts doesn't work because it must be edge-runtime compliant and it relies on "fs" to open the .env files. I suspect this would raise issues with bundling NEXT_PUBLIC variables.

Calling this function from next.config.js doesn't seem to work.

const projectDir = process.cwd()
// loading "../.env"
const res = loadEnvConfig(path.resolve(projectDir, ".."))
// res will have the right loadedEnvFiles, 
// but not the right combinedEnv value, our TEST_VAR value is missing
console.log(path.resolve(projectDir, ".."), res, process.env.TEST_VAR)

Vite does have an envDir option for this use case.

Proposal

Support an "envDir" variable in next.config.js, that can point to the root folder: envDir: "../".