Installed and configured the logstash-filter-json filter for message parsing in JSON out of the box.

logstash/Dockerfile

@@ -1 +1,4 @@
-FROM logstash:latest
+FROM logstash:latest
+
+# Install plugins
+RUN logstash-plugin install logstash-filter-json

logstash/config/logstash.conf

@@ -6,6 +6,13 @@ input {
 
 ## Add your filters here
 
+# Added a default json filter for parsing the "message" field
+filter {
+    json {
+        source => "message"
+    }
+}
+
 output {
 	elasticsearch {
 		hosts => "elasticsearch:9200"