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/docker-compose.yml

@@ -0,0 +1,11 @@
+version: "3.6"
+services:
+
+  sat:
+    image: salutatoi/sat
+    build: backend
+
+  libervia:
+    build: libervia
+    depends_on:
+      - sat