docker: update duniter image and use volumes
- update duniter image (to last pini's unofficial one)
- use docker volumes
- fix typo in documentation
Gitlab was migrated to a new server. Everything should be back to normal and no visible change should remain. If you see anything wrong, please report them to https://forum.duniter.org/t/interventions-et-indisponibilite-des-forums-et-de-gitlab/7131/271