feat: selectively trigger PR docker-builds workflow for external forks

[rcmorano]

Is your feature request related to a problem? Please describe.

As commented in this PR, we should be extra careful triggering pipelines with secrets for external forks, which is currently the case for docker-builds workflow.

Describe the solution you'd like

We should think about some strategy to manually trigger/selectively trigger them, ie, only when they are labeled.

Describe alternatives you've considered

No response

Additional context

[iFergal]

I think there's just a github option where it never starts until someone with powers clicks go.

I believe in WebOfTrust this was the case for me, at least until I had my first PR merged. But I would go stricter than that if we can, because easy to get a first PR merged if it's minimal changes.

[rcmorano]

I think there's just a github option where it never starts until someone with powers clicks go.

I believe in WebOfTrust this was the case for me, at least until I had my first PR merged. But I would go stricter than that if we can, because easy to get a first PR merged if it's minimal changes.

AFAIK there is no strict button for that, but I think we can make it implicit to PR approvals with this event.