Make gatsbyjs build fast

Because build is not incremental, one way to make gatsbyjs build fast is to keep as much thing you can in cache .

One way to do this in Amplify Console is:

    build:
      commands:
        - nvm use $VERSION_NODE_12
        - yarn run build
  artifacts:
    baseDirectory: gatsby/public
    files:
      - '**/*'
  cache:
    paths:
      - gatsby/public/static/**/*
      - gatsby/node_modules/**/*