TestAgentContainerSuite/TestDockerAgent

## Flaky Test

* **Test Name:** TestAgentContainerSuite/TestDockerAgent
* **Link:** [Link to file/line number in github.](https://github.com/elastic/fleet-server/blob/81895f995eff40c2deef12c1f09df9ba072d05f1/testing/e2e/agent_container_test.go#L297)
* **Branch:** 9.0
* **Notes:**

https://buildkite.com/elastic/fleet-server/builds/11014#0199aa31-2d04-41df-932c-5e7c472dad0a/7-72

### Stack Trace

```
agent_container_test.go:417:
Error Trace: /opt/buildkite-agent/builds/bk-agent-prod-gcp-1759497515604663238/elastic/fleet-server/testing/e2e/agent_container_test.go:417
/opt/buildkite-agent/.gvm/versions/go1.25.1.linux.amd64/src/runtime/asm_amd64.s:1693
Error: Received unexpected error:
context deadline exceeded
Test: TestAgentContainerSuite/TestDockerAgent
Messages: context expired before agent reported online
agent_container_test.go:400: elastic-agent container logs (read err: ):
agent container initialisation - effective capabilities
agent container initialisation - ambient capabilities
agent container initialisation - chown paths
{"log.level":"info","@timestamp":"2025-10-03T13:31:23.006Z","log.origin":{"function":"github.com/elastic/elastic-agent/internal/pkg/agent/cmd.(*enrollCmd).enrollWithBackoff","file.name":"cmd/enroll_cmd.go","file.line":534},"message":"Starting enrollment to URL: https://fleet-server:8220/","ecs.version":"1.6.0"}
{"log.level":"info","@timestamp":"2025-10-03T13:31:23.210Z","log.origin":{"function":"github.com/elastic/elastic-agent/internal/pkg/agent/cmd.(*enrollCmd).enrollWithBackoff","file.name":"cmd/enroll_cmd.go","file.line":540},"message":"1st enrollment attempt failed, retrying enrolling to URL: https://fleet-server:8220/ with exponential backoff (init 5s, max 10m0s)","ecs.version":"1.6.0"}
{"log.level":"warn","@timestamp":"2025-10-03T13:31:23.210Z","log.origin":{"function":"github.com/elastic/elastic-agent/internal/pkg/agent/cmd.(*enrollCmd).enrollWithBackoff","file.name":"cmd/enroll_cmd.go","file.line":559},"message":"Enrollment failed: fail to execute request to fleet-server: dial tcp 172.18.0.6:8220: connect: connection refused","ecs.version":"1.6.0"}
Error: fail to enroll: fail to execute request to fleet-server: dial tcp 172.18.0.6:8220: connect: connection refused
For help, please see our troubleshooting guide at https://www.elastic.co/guide/en/fleet/9.0/fleet-troubleshooting.html
Error: enrollment failed: exit status 1
For help, please see our troubleshooting guide at https://www.elastic.co/guide/en/fleet/9.0/fleet-troubleshooting.html
```