docker: dockers images, first draft:

This patch introduce new docker images, replacing the unmaintained ones which were in
sat_docs.

Images + docker-compose files are provided to use SàT backend and Libervia, but also to
integrate and configure Prosody and SàT PubSub to run end-to-end tests.

docker/backend/scripts/entrypoint.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+
+eval $(dbus-launch --sh-syntax)
+
+# default, backend is launched in foreground
+if [ $# -eq 0 ]
+then
+	exec sat fg
+fi
+
+# backend is launched with an explicit subcommand
+if [ $1 = fg -o $1 = bg -o $1 = debug -o $1 = stop -o $1 = status ]
+then
+	exec sat "$@"
+fi
+
+# a whole command is specified
+sat bg
+exec "$@"