gitlab-ci: Automatically run CI checks for all applicable GitHub PRs

[kmk3]

Pipelines were executed for pull requests that have no relevance to GitLab CI,
such as the following ones about updating the versions of GitHub Actions:

• build(deps): bump actions/checkout from 3.0.1 to 3.0.2 #5119
• build(deps): bump github/codeql-action from 2.1.8 to 2.1.9 #5128
• build(deps): bump github/codeql-action from 2.1.9 to 2.1.10 #5144

->

• https://gitlab.com/Firejail/firejail_ci/-/pipelines/523611105
• https://gitlab.com/Firejail/firejail_ci/-/pipelines/529088107
• https://gitlab.com/Firejail/firejail_ci/-/pipelines/539786701

But they were not executed for the following pull requests that actually change
the code and/or .gitlab-ci.yml:

• fzenity: fix dead store #5134
• ci: fix --version not printing compile-time features #5147
• ci: print version after install & fix apparmor support on build_apparmor #5148

@Fred-Barclay @reinerh

[kmk3]

Also, can someone add me (same username) to the GitLab project so that I can
debug the CI there?

[reinerh]

Also, can someone add me (same username) to the GitLab project so that I can debug the CI there?

invite sent.

[kmk3]

@reinerh commented on May 17:

Also, can someone add me (same username) to the GitLab project so that I
can debug the CI there?

invite sent.

Thanks!

[kmk3]

Regarding the PRs, it looks like the default rule on GitLab CI is something
like "always build on (first-party) branch creation/modification", which by
default would include master and any other branches that exist in this
repository (and any branch that is pushed only to the mirror).

Since dependabot creates the branches directly in this repository (rather than
in a fork), all of its branches also get tested. But branches from forks are
not copied over to the GitLab mirror when a PR is opened, so they are not
tested.

So it should be simple to ignore dependabot PRs based on paths changed or on
branch name, but automatically building PRs opened from forks seems trickier.

I see now that this feature is actually being tracked by GitLab:

Main issue:

• Backend: Allow CI/CD mirroring to handle forks on GitHub SCM
  (2018-04-13)

Related issues:

• Fetch refs/pull/*/head refs for mirror repository (2019-09-05)
• Spike: Allow CI/CD mirroring to handle forks on GitHub SCM issue
  (2020-11-12)

To anyone reading this, please consider voting on the main issue if you think
it would be useful (it's at +77/-0 votes as of writing).