chore: add wp-cli maintenance override

docker-compose.maintenance.yml

@@ -0,0 +1,10 @@
+# Optional maintenance override.
+#
+# The default wp-cli service only joins the internal data network. Use this override
+# when a controlled maintenance command must reach the internet, for example a
+# WordPress core download from wordpress.org.
+services:
+  wp-cli:
+    networks:
+      - data
+      - web