title: Gitlab-ci and docker - env variables
body: <p>My scenario: Gitlab runner deploying my react app in ubuntu server using docker.</p>
  <p>My problem: I want to know how can i pass the environmental variables created in gitlab to docker compose ( or dockerfile). In order to use them when building the app.</p>
  <p>My files:</p>
  <pre><code>image: docker:latest
  services:
      - docker:dind
  
  stages:
      - deploy
  
  step-develop:
      stage: deploy
      only:
          - dev
      tags:
          - dev
      script:
          - echo MY_VAR_FROM_GITLAB # This is working good.
          - sudo docker image prune -f
          - sudo docker-compose -f docker-compose.yml build --no-cache
          - sudo docker-compose -f docker-compose.yml up -d
  step-production:
      stage: deploy
      only:
          - prod
      tags:
          - prod
      script:
          - sudo docker image prune -f
          - sudo docker-compose -f docker-compose-prod.yml build --no-cache
          - sudo docker-compose -f docker-compose-prod.yml up -d
  </code></pre>
  <pre><code>#the docker compose file version
  version: '3'
  # you can run multiple services inside one docker compose file
  # define them with their dependencies one after the other
  services:
      # service 1 named react-dev
      react-dev:
          # service 1 container name
          container_name: react-dev
          build:
              # the context (working directory) is the current directory
              # change this to the directory containing the dockerfile if in a different place
              context: .
              # the dockerfile to be run
              dockerfile: Dockerfile
          # map the exposed port from the underlying service to a port exposed to the outside
          # in this case  map port 3000 exposed by create react app to also 3000
          # to be used to access the container from the outside
          ports:
              - '3000:80'
          # the mounted volumes (folders which are outside docker but being used by docker)
          # volumes:
          #     - '.:/gt-strapi-react'
          #     - '/gt-strapi-react/node_modules'
          # set the environment to development
          environment:
              - NODE_ENV=development
  </code></pre>