Setup Gitlab CI/CD environment variables via UI

I am new at gitlab CI/CD Settings. I don't want to put sensitive credentials (like API-Keys, passwords...) into my branch. For this, GitLab (and other CI/CD-Services) are able to set environment variables.

What I have done so far:

1. Via UI (Project ⇒ Settings ⇒ CI/CD ⇒ Variables)

   First go to Project ⇒ Settings ⇒ CI/CD ⇒ Variables and add them like this: enter image description here

   Now here trying to get the File with all your config-values(e.g. with dotenv).

   require("dotenv");
   module.exports = process.env.NODE_ENV.trim() === "production" ? _config.production : _config.development;
   

Current .gitlab-ci.yaml file is:

image: node:8.9.0

cache:
  paths:
   - node_modules/

stages:
 - ver
 - init
 - test
 - build
 - deploy

 ver:
   stage: ver
   script:
   - node -v

init:
  stage: init-dev
  script:
   - npm install
  tags:
   - dev_server
  only:
   - dev  
  variables:
    ENV_PRODUCTION: "/builds/AkdiD/8/abcde/projectName/ENV_PRODUCTION" 
   
test:
   stage: test
   script:
    - npm test

build:
 stage: build
 script:
  - echo "BUILD_VERSION=production" >> build.env
 artifacts:
   reports:
   dotenv: build.env   

 deploy:
   stage: deploy-dev
   script:
    - npm run killcurrent
    - echo $ENV_PRODUCTION
    - echo $BUILD_VERSION
    - npm run staging
   tags:
    - dev_server
   only:
    - dev

Question: where do I need to keep this ENV_PRODUCTION file name (yaml file or other place) so that server take that value ??

Edited variable like this- still server it not fetching these variables. Should I change/put something in .gitlab-ci.yml file?