docker (demo): fix starting + better UX:

- Use the new `wait_for_service` option to be sure that Prosody is launched.
- Use the new `init_script_path` option to initiate forums and bookmark a demo room.
- Activate MUC service.
- Change log formats to make them more readable.

docker/backend-dev-demo/forums.json

@@ -0,0 +1,38 @@
+[
+    {
+        "main-language": "en",
+        "name": "Libervia",
+        "title": "Libervia",
+        "short-desc": "Forums about Libervia demo",
+        "sub-forums": [
+            {
+                "uri": "",
+                "title": "tools and features",
+                "short-desc": "Discussions about Libervia frontends, tools, features, or changes"
+            },
+            {
+                "uri": "",
+                "title": "questions and help",
+                "short-desc": "Mutual assistance for installation or using Libervia and its ecosystem"
+            }
+        ],
+        "desc": "Those are example forums."
+    },
+    {
+        "main-language": "en",
+        "title": "XMPP and associated",
+        "sub-forums": [
+            {
+                "uri": "",
+                "title": "XMPP software",
+                "short-desc": "Topics about XMPP software other than Libervia"
+            },
+            {
+                "uri": "",
+                "title": "outside XMPP",
+                "short-desc": "You can discuss or present here protocols or software similar to XMPP"
+            }
+        ],
+        "desc": "Forums about XMPP in general, or similar protocols"
+    }
+]