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

doc/dev/git-conventions.md

@@ -28,7 +28,7 @@ The **action** must be a verb in imperative form, the **subject** a noun.
 
 For example, we rename the trait `Foo` to `Fii` in the `toto` module:
 
-    [ref] toto: rename Foo -> Fii
+    ref(toto): rename Foo -> Fii
 
 Commits must be lowercase.