allow to define trigger-project with a variable in multi-project pipelines trigger

Proposal

Dear gitlab team,

I have a paused pipeline when using the CI code below. I can use variable in a script job, and I would like to do the same in a trigger job.

# I can echo the variable in a script job
.trigger_script: 
  stage: trigger
  only:
    - feature-multi_project_pipelines
  script: 
    - echo container-images/${project_name}

bridge script: 
  extends: .trigger_script
  variables:
    project_name: puppet

# I cannot use a variable in a trigger job
.trigger:
  stage: trigger
  only:
    - feature-multi_project_pipelines
  trigger: 
    project: container-images/${project_name}
    branch: master

bridge: 
  extends: .trigger
  variables:
    project_name: puppet

I simplify the code so it is readable here. But, the point is that I will have a lot of bridge jobs extending '.trigger' (that will be included) and I woul like to give 'project_name' as variable to hide/mutualize complexity.