feat(ext/fleet): Store state in a volume

Prevents duplicate enrollments when the fleet-server container is
re-created.

extensions/fleet/Dockerfile

@@ -1,3 +1,8 @@
 ARG ELASTIC_VERSION
 
 FROM docker.elastic.co/beats/elastic-agent:${ELASTIC_VERSION}
+
+# Ensure the 'state' directory exists and is owned by the 'elastic-agent' user,
+# otherwise mounting a named volume in that location creates a directory owned
+# by root:root which the 'elastic-agent' user isn't allowed to write to.
+RUN mkdir state

extensions/fleet/README.md

@@ -33,7 +33,6 @@ management UI: [Fleet UI Settings][fleet-cfg].
 
 ## Known Issues
 
-- Every re-creation of the `fleet-server` container creates a duplicate agent in Fleet's central management.
 - Logs and metrics are only collected within the Fleet Server's container. Ultimately, we want to emulate the behaviour
   of the existing Metricsbeat and Filebeat extensions, and collect logs and metrics from all ELK containers
   out-of-the-box. Unfortunately, this kind of use-case isn't (yet) well supported by Fleet, and most advanced

extensions/fleet/fleet-compose.yml

@@ -6,6 +6,8 @@ services:
       context: extensions/fleet/
       args:
         ELASTIC_VERSION: ${ELASTIC_VERSION}
+    volumes:
+      - fleet-server:/usr/share/elastic-agent/state:z
     environment:
       FLEET_SERVER_ENABLE: '1'
       FLEET_SERVER_INSECURE_HTTP: 'true'
@@ -28,3 +30,6 @@ services:
     depends_on:
       - elasticsearch
       - kibana
+
+volumes:
+  fleet-server: