====== dynamic runner tag ======

Variable expansion in tags works. 

Simple runner tag can be set as shown:

<code yaml>
variables:
  RUNNER_TAG: "staging"
job:
  tags:
    - $RUNNER_TAG  
</code>

<code yaml>
tags:
  - ${CI_COMMIT_BRANCH}
</code>

Not possible to define array of strings as variable:
<code yaml>
variables:
  RUNNER_TAG: [ "staging", "test" ]   # LINT ERROR
job:
  tags: $RUNNER_TAGS
</code>

[[https://stackoverflow.com/questions/65850715/how-to-set-a-dynamic-gitlab-job-tag-with-a-variable|How to set a dynamic gitlab job tag with a variable?]]

===== run pipeline on the same runner =====

It is not possible to detect runner tag during yaml variables expansion because gitlab doesn't schedule jobs. Gitlab-runners periodically connect to Gitlab servers
and asks for jobs. 

It is possible to check what runner got our job from environment variable:

<code yaml>CI_RUNNER_TAGS=["linux", "docker", "small", "medium"]</code>

and this variable can be used to generate yaml file for new dynamically generated child pipeline.