Build from subfolder

Building from subfolder (from Amplify's perspective) is same as a Monorepo.

Below code would build from /frontend/*:

version: 1
applications:
  - appRoot: frontend
    frontend:
      phases:
        preBuild:
          commands:
            - yarn install --frozen-lockfile
        build:
          commands:
            - yarn run build
      artifacts:
        baseDirectory: .next
        files:
          - '**/*'
      cache:
        paths:
          - .next/cache/**/*
          - node_modules/**/*

Environment variable AMPLIFY_MONOREPO_APP_ROOT must also be added to the Environment Variables

If your project has a flattened structure (meaning, when the entrypoint is at the root of the repository), you need to point both the appRoot and the baseDirectory to ..

version: 1
applications:
  - frontend:
      buildPath: '.'
      phases:
        preBuild:
          commands:
            - yarn install --frozen-lockfile
        build:
          commands:
            - yarn run build
      artifacts:
        baseDirectory: .next
        files:
          - '**/*'
      cache:
        paths:
          - .next/cache/**/*
          - node_modules/**/*
    appRoot: '.'