Feature: Make it possible to trigger special pipelines (like `CRON_MODE=…`) from special commits

Ideally, as a docker-coq maintainer or so, we should be able to run:

git commit --allow-empty -m "chore: …" -m "docker-keeper: rebuild-all"
or
git commit --allow-empty -m "chore(docker-keeper): rebuild-all"
or
git commit --allow-empty -m "chore: docker-keeper: rebuild-all"

(IDK which phrasing would be better)

then

git push

to trigger the expected pipelines.