Gitlab-ci variable interpolation

In gitlab-ci I have two files stored in production/configurations.yml and staging/configurations.yml, folders. They hold the configurations for each environment.

I would like to include the file as per the value informed by the users when manually running the pipelines( either production or staging). However, I couldn't work out how to do that, as the variable will only be available when the job runs, until then it is null, therefore the validation fails with:

"This GitLab CI configuration is invalid: Local file configurations.yml does not exist!"

stages:
 - build
 - deploy
 
variables:
  DEPLOY_ENVIRONMENT:
    value: "production"
    options:
      - "production"
      - "staging"

    description: "The deployment target. Set to 'staging' by default."
  DEPLOY_COLOUR:
    value: "blue"
    options:
      - "blue"
      - "green"
    description: "The deployment target. Set to 'blue' by default."    
  DEPLOY_VERSION:
    value: ""
    description: "The deployment version. e.g. 1.0.0 for production && 1.0.0-rc for staging."   
  DEPLOY_APPLICATION:
    value: "app1"
    options:
      - "app1"
      - "app2"
    description: "The application to deploy"   

  

include: 
 - local: $DEPLOY_ENVIRONMENT/configurations.yml
 

Update_GIT:
  stage: build
...

is that even possible? Any ideas/workarounds, please?

Gitlab has a predefined variable CI_COMMIT_BRANCH that is the name of the user's commit branch. Assuming your users have a common naming convention for staging and production branches (i.e. "develop", "main", "prod" etc.) you can use workflow:rules:variables to set the variable according to which branch the user is running through the pipeline.

Example stolen from Gitlab documentation:

workflow:
  rules:
    - if: $CI_COMMIT_BRANCH == "develop" || $CI_COMMIT_BRANCH == "test"
      variables:
        DEPLOY_ENVIRONMENT: "staging" 
    - if: $CI_COMMIT_BRANCH == "prod" || $CI_COMMIT_BRANCH == "master"
      variables:
        DEPLOY_ENVIRONMENT: "production" 

All of Gitlab's predefined variables, including CI_COMMIT_BRANCH and their uses can be found here.

EDIT:

Sorry, I didn't see the part of your question where users manually set the variable. I understand now. Perhaps not the most elegant solution, but you can introduce rules to your include to be dependent on DEPLOY_ENVIRONMENT

include:
  - local: staging/configuration.yml
    rules:
      - if: $DEPLOY_ENVIRONMENT == "staging"
  - local: production/configuration.yml
    rules:
      - if: $DEPLOY_ENVIRONMENT == "production"

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