Sending requests to cluster-listen-address of AlertManager

[Alexandre Lamarre]

I am spinning up an AlertManager instace with bitnami's docker image :

`docker container run --name alertmanager --rm -p 9094:9094 bitnami/alertmanager`

The other day I was able to use the /api/v2 of the cluster listen address, however now I am getting `Connection was forcibly closed by a peer` . The ports are not already in use.

Bitnami's config file is :
```
route:
  group_by: ['alertname']
  group_wait: 30s
  group_interval: 5m
  repeat_interval: 1h
  receiver: 'web.hook'
receivers:
  - name: 'web.hook'
    webhook_configs:
      - url: 'http://127.0.0.1:5001/'
inhibit_rules:
  - source_match:
      severity: 'critical'
    target_match:
      severity: 'warning'
    equal: ['alertname', 'dev', 'instance']
```


Similarly when I start up this instance with the alertmanager binary on an arbitrary port, I can get the web api, but when I try running an http req against the cluster-listen-address I still get an EOF (Connection closed by peer)


```
Expected success, but got an error:
      <*url.Error | 0xc0008bbe90>: {
          Op: "Get",
          URL: "http://127.0.0.1:35269/api/v2/receivers",
          Err: <*errors.errorString | 0xc0001180f0>{s: "EOF"},
      }
```

Any ideas what's going wrong? Was working fine the other day