Add docs for enrollment handling for containerized agents (#729)

This is a forward fit of
elastic/ingest-docs#1728 into the 9.0 docs.

See: [preview
page](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/729/reference/ingestion-tools/fleet/enrollment-handling-containerized-agent)

Author: kilfoyle

reference/ingestion-tools/fleet/enrollment-handling-containerized-agent.md

@@ -0,0 +1,15 @@
+# Enrollment handing for containerized agents
+
+For {{fleet}}-managed {{agents}} that run in a containerized environment (including Docker, Kubernetes, and others), enrollment handling is managed as follows:
+
+* **Enrollment Verification:** {{agent}} checks the stored enrollment conditions within its container environment and re-enrolls only when necessary.
+* **Unenrollment Handling:** If an {{agent}} is unenrolled via the {{fleet}} UI but still references a valid enrollment token provided through environment variables, it will automatically re-enroll on the next container restart.
+
+In versions lower than 8.18, an unenrolled agent remains unenrolled and does not re-enroll, even if a valid enrollment token is still available.
+
+
+
+
+
+
+

reference/ingestion-tools/fleet/install-elastic-agents-in-containers.md

@@ -23,6 +23,10 @@ To learn how to run {{agent}}s in a containerized environment, see:
 * [Using a custom ingest pipeline with the {{k8s}} Integration](/reference/ingestion-tools/fleet/ingest-pipeline-kubernetes.md)
 * [Run {{agent}} on ECK](/deploy-manage/deploy/cloud-on-k8s/standalone-elastic-agent.md) — for {{eck}} users
 
+::::{note}
+Enrollment handling for {{agent}} in a containerized environment has some special nuances.
+For details refer to [Enrollment handing for containerized agents](./enrollment-handling-containerized-agent.md).
+::::
 
 
 

reference/ingestion-tools/fleet/unenroll-elastic-agent.md

@@ -20,4 +20,7 @@ Unenrolled agents will continue to run, but will not be able to send data. They
 If unenrollment hangs, select **Force unenroll** to invalidate all API keys related to the agent and change the status to `inactive` so that the agent no longer appears in {{fleet}}.
 ::::
 
-
+::::{note}
+Enrollment handling for {{agent}} in a containerized environment has some special nuances.
+For details refer to [Enrollment handing for containerized agents](./enrollment-handling-containerized-agent.md).
+::::

reference/toc.yml

@@ -104,6 +104,7 @@ toc:
                   - file: ingestion-tools/fleet/monitor-elastic-agent.md
                   - file: ingestion-tools/fleet/agent-health-status.md
                   - file: ingestion-tools/fleet/filter-agent-list-by-tags.md
+                  - file: ingestion-tools/fleet/enrollment-handling-containerized-agent.md
               - file: ingestion-tools/fleet/agent-policy.md
                 children:
                   - file: ingestion-tools/fleet/create-policy-no-ui.md