Add support for validating multiple local CI configuration files at once

Note from maintainers: This issue describes a problem with CI config files that depend on the context set by the main .gitlab-ci.yml.

The best solution reached in the comments is to allow to validate multiple local files at once. "Local" meaning file from local repository folder, even if it's changed or not commited. From the example below, that means including the local ci/jobs.yaml and ci/templates.yaml with the request to validate the main .gitlab-ci.yml. This feature is blocked by the lack of GitLab API support: gitlab#349696

original title

Feature request: Gitlab CI validation consider all related yaml files during validation

Problem to solve

I am using Gitlab vscode extension to validate my Gitlab CI yaml files, the extension currently is only validate the current file without involving all the extended files. Therefore I often get "Job_1 : Unknown keys in extends (template_1)"

Proposal

If we can search for all the extended files based on the include block in the .gitlab-ci.yml file first, then validate the specific file, it will make the validation more legitimate.

Further details

For example, I have a .gitlab-ci-yml like following:

include:
  - local: "ci/jobs.yaml"
  - local: "ci/templates.yaml"

In the jobs.yaml file, I have a lot of jobs extends the template in templates.yaml file:

job_1:
  extends: temp_1
  variables:
    - bla

job_2:
  extends: temp_2
  variables:
    - bla

And in templates.yaml file, I have those templates defined.

Now I want to validate jobs.yaml file, but I got the validation saying that they don't recognize temp_1 and temp_2. If the extension is able to find the templates.yaml by using the .gitlab-ci.yaml file, then we can validate deeper on the jobs.yaml file. This might require users to config the location of the .gitlab-ci.yaml file in the extension or the extension will look for them in the root of the project automatically.

Links / references

The Gitlab CI Lint has this feature: https://docs.gitlab.com/ee/ci/lint.html, but every time when I use it I have to manually paste the include block into the tool as well.