docker (backend-dev): move dbus unix socket: there seem to be a regression with Docker: the volume is not propagated anymore with pre-existing files when a unix socket is inside. This is the case in `backend-dev` with the D-Bus unix socket put in `/home/libervia/.local/share/libervia/dbus_socket`, when the volume is mounted at `/home/libervia/.local/share/libervia/`, resulting in hard to debug failures. To work around that, the socket is not put in the volume mount point anymore, and has been moved to `/home/libervia/.local/share/`

[DEFAULT]
media_dir = /src/libervia-media