[CI Failure] Flaky: WithDockerfileTests.AddDockerfileLaunchesContainerSuccessfully times out in DcpResourceWatcher

## Build Information

Build: https://github.com/microsoft/aspire/actions/runs/30987535630
Build error leg or test failing: Tests / Hosting.Containers / Hosting.Containers (ubuntu-latest) / `Aspire.Hosting.Containers.Tests.WithDockerfileTests.AddDockerfileLaunchesContainerSuccessfully`
Pull request: #18989

## Error Message

```
Polly.Timeout.TimeoutRejectedException : The operation didn't complete within the allowed timeout of '00:00:20'.
---- System.Threading.Tasks.TaskCanceledException : A task was canceled.
at Polly.Timeout.TimeoutResilienceStrategy.ExecuteCore
at Aspire.Hosting.Dcp.DcpResourceWatcher DcpResourceWatcher.cs:line 173
Hosting failed to start
```

## Description

Flaky: WithDockerfileTests.AddDockerfileLaunchesContainerSuccessfully times out in DcpResourceWatcher

**Type**: flaky-test

## Occurrences

| Date | Build | Job | PR |
|------|-------|-----|----|
| 2026-08-05 | [30987535630](https://github.com/microsoft/aspire/actions/runs/30987535630) | Tests / Hosting.Containers / Hosting.Containers (ubuntu-latest) | #18989 |

Start with Aspire.Hosting.Containers.Tests.WithDockerfileTests.AddDockerfileLaunchesContainerSuccessfully and the failing Hosting.Containers build from run 30987535630. Read DcpResourceWatcher.cs around line 173 and investigate why the 20-second operation times out during container startup. Done means the test no longer flakes or times out in repeated CI runs.

Written by the indexing model from the issue text.