Feature proposal: Variable expansion in deployment_tier

Proposal

Support variable expansion in the environment:deployment_tier field of a job in the .gitlab-ci.yml.

Use Case

When a child pipeline is triggerd (dynamic or static) and then used to deploy to multiple environments.

.gitlab-ci-deploy.yml

service-a:
  environment:
    name: $ENVIRONMENT/service-a
    deployment_tier: $DEPLOYMENT_TIER
  script:
    - ./deploy.sh

.gitlab-ci.yml

  
deploy:
  variables:
    ENVIRONMENT: nonprod
    DEPLOYMENT_TIER: staging
  trigger:
    include: .gitlab-ci-deploy.yml

deploy-prod:
  variables:
    ENVIRONMENT: prod
    DEPLOYMENT_TIER: production
  trigger:
    include: .gitlab-ci-deploy.yml

Implementation

The code for fetching the tier is in Ci::Deployable#environment_tier_from_options. This needs to be updated to use ExpandVariables on the configured value. See #expanded_environment_name and #expanded_kubernetes_namespace for existing examples.

In order to preserve validation integrity, this blocked until GitLab has upgraded to Rails 7.1, see #365402 (comment 2200490852) for details. (Ref: Blocked by the apic Rails 7.1 upgrade (&11470))