feat: add initial Docker Compose infrastructure

infra/docker/backend/placeholder_wsgi.py

@@ -0,0 +1,10 @@
+def application(environ, start_response):
+    body = b"AzioneLab backend placeholder\n"
+    start_response(
+        "503 Service Unavailable",
+        [
+            ("Content-Type", "text/plain; charset=utf-8"),
+            ("Content-Length", str(len(body))),
+        ],
+    )
+    return [body]