Fix mattermost docker-compose example

example/docker-compose-mattermost/config.yaml

@@ -1,6 +1,16 @@
+alerting:
+  mattermost:
+    webhook-url: "http://mattermost:8065/hooks/tokengoeshere"
+    insecure: true
+
 services:
   - name: example
     url: http://example.org
-    interval: 30s
+    interval: 1m
+    alerts:
+      - type: mattermost
+        enabled: true
+        description: "healthcheck failed 3 times in a row"
+        send-on-resolved: true
     conditions:
       - "[STATUS] == 200"

example/docker-compose-mattermost/docker-compose.yml

@@ -1,13 +1,24 @@
 version: "3.8"
+
 services:
   gatus:
+    container_name: gatus
     image: twinproduction/gatus:latest
     ports:
       - 8080:8080
     volumes:
       - ./config.yaml:/config/config.yaml
+    networks:
+      - default
 
-    mattermost-preview:
-        image: mattermost/mattermost-preview:latest
+  mattermost:
+    container_name: mattermost
+    image: mattermost/mattermost-preview:5.26.0
     ports:
       - 8065:8065
+    networks:
+      - default
+
+networks:
+  default:
+    driver: bridge