docker: include .gitignore into .dockerignore
This is to prevent the result of previous in place builds to interfere with the docker image build.
Gitlab was upgraded to latest version (v18.3.1), please report issues to https://forum.duniter.org/t/interventions-et-indisponibilite-des-forums-et-de-gitlab/7131/298
This is to prevent the result of previous in place builds to interfere with the docker image build.