docker: Mount and clone local swh-jenkins-jobs repo for conf testing
Currently, the only way to test Jenkins configuration changes using docker is to push changes to a remote feature branch in swh-jenkins-jobs repository and make jenkins clone that branch to test the changes.
This is not really convenient to quickly test changes so prefer to mount the local swh-jenkins-jobs repository as a docker volume and clone that repository from Jenkins filesystem.
Migrated from D8234 (view on Phabricator)