Feature: Make it possible to ease propagation of rebuilds from (docker-base to) docker-coq to docker-mathcomp

Related: wishlist #1 (closed)

Related: after_deploy:, after_deploy_export: (maybe enough, maybe not)

Main concern: avoid redundant webhooks

Maybe: extend GitLab CI notify stage

Maybe: think about cases where propagation shouldn't be triggered ; if there can be some, provide an option