Gitlab CI/CD: Setup variables relating to environment

I am working with gitlab version 14.10.5.

In the left panel, there is a "Deployments" icone that allow me to create environments. In a gitlab repository I want to set up pipeline for dev, stage and main branches relating to 3 environments.

In these environments, I want to setup specific variables that will chaneg over environments.

For example, I want the variable NINJA to be setup with the value:

• "1" for the branch dev
• "2" for the branch stage
• "3" for the branch main

So, in my .gitlab-ci.yml I have:

build-testing:
  stage: build
  script:
    - echo "Hello team"
    - echo "$NINJA"
  environment:
    name: testing
  only:
    refs:
      - dev


build-staging:
  stage: build
  script:
    - echo "Hello team"
    - echo "$NINJA"
  environment:
    name: staging
  only:
    refs:
      - stage


build-production:
  stage: build
  script:
    - echo "Hello team"
    - echo "$NINJA"
  environment:
    name: production
  only:
    refs:
      - main

How to setup the variable NINJA in an environment, in gitlab ?