docker-compose: add blackhole receiver for alertmanager (#999)

Currently, alertmanager spams logs with `Notify attempt failed, will retry later` message
because default receiver is unreachable. The change updates default configuration with
blackhole receiver which means alertmanager will continue to accept alerts but won't make
attempts to send them anywhere.

https://github.com/VictoriaMetrics/VictoriaMetrics/issues/995

deployment/docker/alertmanager.yml

@@ -0,0 +1,5 @@
+route:
+  receiver: blackhole
+
+receivers:
+  - name: blackhole

deployment/docker/docker-compose.yml

@@ -77,6 +77,10 @@ services:
   alertmanager:
     container_name: alertmanager
     image:  prom/alertmanager
+    volumes:
+      - ./alertmanager.yml:/config/alertmanager.yml
+    command:
+      - '--config.file=/config/alertmanager.yml'
     ports:
       - 9093:9093
     networks: