docker: set `DBUS_SESSION_BUS_ADDRESS` so that it uses remote TCP with local fallback: When connecting to Docker service, first the `backend` container will be tried with TCP, then an Unix Socket will be used as fallback. The Unix Socket is put in Libervia local dir which is a volume shared between containers. This way D-Bus can be used with a remote backend, or the backend can be run locally (notably during build when accounts creation or other settings must be done via Libervia CLI).

[DEFAULT]
media_dir = /src/libervia-media

[cli]
# FIXME: default "auto" value cause trouble
background = dark