Rules for setting build image runner tags don't merge properly

Summary

The refactoring of the definition of custom runner tags to build images in !453 (merged) didn't account for the proper merging of rules for the gitlab-opentofu-image:build: and gitlab-opentofu-image:build:rootless: jobs to reproduce the original rules pre-refactoring. As a consequence, the forked component in our enterprise instance of Gitlab isn't able to resolve the pipeline as of said MR, failing with:

Unable to create pipeline gitlab-opentofu-image:build: unknown keys in extends (.runner-tags:build-image).

Steps to reproduce

  1. Install an enterprise instance of Gitlab.
  2. Fork the OpenTofu CI/CD component.
  3. Try to run the pipeline

Example Project

What is the current bug behavior?

The gitlab-opentofu-image:build job fails with:

Unable to create pipeline gitlab-opentofu-image:build: unknown keys in extends (.runner-tags:build-image).

What is the expected correct behavior?

The gitlab-opentofu-image:build job should be able to run, as per the rules pre-MR.

Relevant logs and/or screenshots

Screenshot with the pipeline outcome in our fork:

Screenshot_2025-12-04_at_17.39.20

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info 

 (For installations with omnibus-gitlab package run and paste the output of: \`sudo gitlab-rake gitlab:env:info\`)  (For installations from source run and paste the output of: \`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production\`)

Results of GitLab application Check

Expand for output related to the GitLab application check 

(For installations with omnibus-gitlab package run and paste the output of: `sudo gitlab-rake gitlab:check SANITIZE=true`)

(For installations from source run and paste the output of: `sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)

(we will only investigate if the tests are passing)

Possible fixes