docs: use latest image tags in examples (#1383)

The README's Docker example and the Docker and Kubernetes examples pointed at `nightly` images, while the README recommends `latest-dind`. Deployments copied from them should run the latest release, not a build of `main`.

None of the examples changed since v5.3.0, so they work as written with the current `latest` tags.

_Written by Claude (Opus 5.5)._

Reviewed-on: https://gitea.com/gitea/runner/pulls/1383
Reviewed-by: Lunny Xiao <[email protected]>
Co-authored-by: silverwind <[email protected]>

examples/kubernetes/rootless-docker.yaml

@@ -70,7 +70,7 @@ spec:
         fsGroup: 1000
       containers:
       - name: runner
-        image: gitea/runner:nightly-dind-rootless
+        image: gitea/runner:latest-dind-rootless
         imagePullPolicy: Always
         env:
         - name: DOCKER_HOST