Provide docs on how to specify which executor is used to run a .gitlab-ci.yml

Page: https://docs.gitlab.com/runner/executors/

Issue: We have docker and shell executors configured. I would like to know how to specify which executor ("tag") is used as the default when executing .gitlab-ci.yml.

I know I can specify this per "job" ie

job-name:
   tags:
     - my-shell-executor

But I am looking for info on now to specify this globally within gitlab-ci.yml