Parse JSON data with filebeat

Elastic Stack Beats
5

I queried Elasticsearch and got the following:

{
        "_index" : "filebeat-2017.03.27",
        "_type" : "log",
        "_id" : "AVsQ0S06i5178hY1Lr4U",
        "_score" : 1.0,
        "_source" : {
          "@timestamp" : "2017-03-27T17:29:16.356Z",
          "beat" : {
            "hostname" : "15f7ccc3a4c1",
            "name" : "15f7ccc3a4c1",
            "version" : "5.2.2"
          },
          "input_type" : "log",
          "message" : "{\"error\":\"dial tcp 172.21.0.2:5432: getsockopt: connection refused\",\"level\":\"error\",\"msg\":\"Could not open database connection\",\"time\":\"2017-03-27T17:19:35Z\"}",
          "offset" : 158,
          "source" : "/mnt/log/error.log",
          "type" : "log"
        }
      }

Could it somehow be related to Docker? I recreated all containers before starting the server and checking the logs. I suppose it doesn't matter which file endings the log files have (.json or .log)?