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).

You'll find install instructions in documentation (see doc/installation.rst).

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