How do I add a .env file in gitlab ci during deployment stage?

So I have a react/typescript app in my repo that I'm working on and in my repo I have a .env file that I'm ignoring so that my secrets don't get exposed and a .env-example file of important environment variables to configure. My problem is, since I'm not pushing the .env file to my repo, when I deploy my app through the google app engine(this is done in the deployment stage in my gitlab-ci.yml file), these environment variables will not be present in production and I need them for my app to work as I do something like this in my webpack.config.js file.

const dotenv = require('dotenv').config({ path: __dirname + '/.env' });

and then

new webpack.DefinePlugin({
  'process.env': dotenv.parsed
})

Here is my .gitlab-ci file for reference in case anyone here wants to see.

cache:
  paths:
    - node_modules/

stages:
  - build
  - test
  - deploy

Build_Site:
  image: node:8-alpine
  stage: build
  script:
    - npm install --progress=false
    - npm run-script build
  artifacts:
    expire_in: 1 week
    paths:
      - build

Run_Tests:
  image: node:8-alpine
  stage: test
  script:
    - npm install --progress=false
    - npm run-script test

Deploy_Production:
  image: google/cloud-sdk:latest
  stage: deploy
  environment: Production
  only:
    - master
  script:
    - echo $DEPLOY_KEY_FILE_PRODUCTION > /tmp/$CI_PIPELINE_ID.json
    - gcloud auth activate-service-account --key-file /tmp/$CI_PIPELINE_ID.json
    - gcloud config set project $PROJECT_ID_PRODUCTION
    - gcloud info
    - gcloud --quiet app deploy
  after_script:
    - rm /tmp/$CI_PIPELINE_ID.json

Also, feel free to critique my gitlab-ci.yml file so I can make it better.

I don't know if you still need this, but this is how I achieved what you wanted to.

1. Create your environment variables in your gitlab repo config.

2. Create setup_env.sh:

   #!/bin/bash
   
   echo API_URL=$API_URL >> .env
   echo NODE_ENV=$NODE_ENV >> .env
   

3. Modify your .gitlab-ci.yml. Upsert below to your before_script: section

     - chmod +x ./setup_env.sh
     - ./setup_env.sh
   

4. In webpack.config.js make use of https://www.npmjs.com/package/dotenv

   require('dotenv').config();
   

   This passes your .env variables available in webpack.config.js file.

5. Add this to your plugins array (add those variables you need):

   new webpack.DefinePlugin({
     'process.env.API_URL': JSON.stringify(process.env.API_URL),
     'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV),
     ...
   })
   

Now your deployment should use your environment variables specified in your gitlab settings.