Docker: move config back to volume /var/lib/duniter and other minor changes

Main change:

• Set back configuration home to /var/lib/duniter/duniter_default. The volume /etc/duniter was created for the keyfile.

Minor changes:

• Merge duniter.sh wrapper into the entrypoint
• Install duniter into /duniter instead of /duniter/duniter
• Doc: fix commit naming convention example
• Entrypoint: run duniter with exec
• Add .dockerignore file