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/`
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).
Using the `DOCKER_LIBERVIA_REV` environment variable, backend (and related repos) revision
can now be specified. By default (variable empty or not set), the current `tip` version is
used, and `dev` is used for tagging. If the variable is set, the revision will be used and
used for tagging the image.
