Consider adding opt-in support to dockerhub based on env vars / secrets

[stronk7]

Pretty much like it was done @ moodle-docker for travis (we were hitting the limits there).

moodlehq/moodle-docker#150

We should do the same here for:

• own moodle-plugin-ci CI (travis and gha).
• recommended (for plugins) templates (travis and gha). Plus document.

Ciao :-)

PS: Note I've remembered about this because I've hit the limits with travis with moodle-plugin-ci, and it was just after 10-12 pushes, so it can happen with relative frequency.

[stronk7]

I was curious about some intense / repetitive / big matrix runs I've performed lately with GHA, because they were not hitting DockerHub limits. And, the very same runs, via TravisCI, are hitting them.

So, after a bit of searching, I found this:

actions/runner-images#1445 (comment)

Basically it seems that Github reached and agreement with Docker and no limits apply to public containers being used by GitHub runners. If private containers or self-hosted runners are used, then credentials may be needed.

So that explains why "normal" jobs aren't hit, so this issue is less critical than I initially was thinking.

Still we have to decide if we want to give any support for those 2 groups (private containers and self-hosted runners).

Ciao :-)