Freeze image sha for dind-sidecar example test.

The example test examples/v1/taskruns/dind-sidecar.yaml has been failing in multiple PRs:
- #7492 (comment)
- #7494 (comment)
- #7458 (comment)

That example has not been updated in a year. My guess is that the [new image](https://hub.docker.com/layers/library/docker/dind/images/sha256-e8c7a73504adaeff4bab939e8d1f18a6114c99c868b9203a66a42f25aa51e613?context=explore) which was released 16 hours ago (at this point) is causing this.

When the image is tagged to the previous version (`docker:24.0.6-dind`) in the taskrun yaml, it executes fine.
This PR updates the example test to use the sha256 of the previously
working image.This PR fixes #7496.

examples/v1/taskruns/dind-sidecar.yaml

@@ -40,7 +40,7 @@ spec:
         name: dind-certs
 
     sidecars:
-    - image: docker:dind
+    - image: docker@sha256:74e78208fc18da48ddf8b569abe21563730845c312130bd0f0b059746a7e10f5
       name: server
       args:
         - --storage-driver=vfs