Add APM Server extension tests

Update extension README

extensions/apm-server/README.md

@@ -1,21 +1,23 @@
 # APM Server extension
 
-Adds a container for Elasticsearch APM server. Forwards caught errors and traces to Elasticsearch to enable their
-visualisation in Kibana.
+The APM Server receives data from APM agents and transforms them into Elasticsearch documents that can be visualised in
+Kibana.
 
 ## Usage
 
-If you want to include the APM server, run Docker Compose from the root of the repository with an additional command
-line argument referencing the `apm-server-compose.yml` file:
+To include APM Server in the stack, run Docker Compose from the root of the repository with an additional command line
+argument referencing the `apm-server-compose.yml` file:
 
 ```console
 $ docker-compose -f docker-compose.yml -f extensions/apm-server/apm-server-compose.yml up
 ```
 
-## Connecting an agent to APM-Server
+Meanwhile, you can navigate to the **APM** application in Kibana and follow the setup instructions to get started.
+
+## Connecting an agent to APM Server
 
 The most basic configuration to send traces to APM server is to specify the `SERVICE_NAME` and `SERVICE_URL`. Here is an
-example Python FLASK configuration:
+example Python Flask configuration:
 
 ```python
 import elasticapm
@@ -36,19 +38,21 @@ app.config['ELASTIC_APM'] = {
 }
 ```
 
-More configuration settings can be found under the **Configuration** section for each language:
-https://www.elastic.co/guide/en/apm/agent/index.html
+More configuration settings can be found for each language in the following documentation page: [Install APM
+agents][apm-agents].
 
 ## Checking connectivity and importing default APM dashboards
 
-From the Kibana Dashboard:
-
-1. `Add APM` button under _Add Data to Kibana_ section
-2. Ignore all the install instructions and press `Check APM Server status` button.
-3. Press `Check agent status`
-4. Press `Load Kibana objects` to get the default dashboards
-5. Lastly press the `APM dashboard` to the bottom right.
+1. On the Kibana home page, click `Add APM` under the _Observability_ panel.
+2. Click `Check APM Server status` to confirm the server is up and running.
+3. Click `Check agent status` to verify your agent has registered properly.
+4. Click `Load Kibana objects` to create an index pattern for APM.
+5. Click `Launch APM` to be taken to the APM dashboard.
 
 ## See also
 
-[Running APM Server on Docker](https://www.elastic.co/guide/en/apm/server/current/running-on-docker.html)
+[Running APM Server on Docker][apm-docker]
+
+
+[apm-agents]: https://www.elastic.co/guide/en/apm/get-started/current/agents.html
+[apm-docker]: https://www.elastic.co/guide/en/apm/server/current/running-on-docker.html

extensions/apm-server/apm-server-compose.yml

@@ -6,10 +6,17 @@ services:
       context: extensions/apm-server/
       args:
         ELK_VERSION: $ELK_VERSION
+    command:
+        # Disable strict permission checking on 'apm-server.yml' configuration file
+        # https://www.elastic.co/guide/en/beats/libbeat/current/config-file-permissions.html
+      - --strict.perms=false
     volumes:
-      - ./extensions/apm-server/config/apm-server.yml:/usr/share/apm-server/apm-server.yml:ro
+      - type: bind
+        source: ./extensions/apm-server/config/apm-server.yml
+        target: /usr/share/apm-server/apm-server.yml
+        read_only: true
     ports:
-      - "8200:8200"
+      - '8200:8200'
     networks:
       - elk
     depends_on: