Update RELEASE.md (#8937)

If opening https://groups.google.com/g/kubernetes-dev we can see the
following message:

  As of January 2, 2022, this group will be sunset in favor of dev@kubernetes.io.

So this replaces kubernetes-dev@googlegroups.com with the new one.

In addition, this adds actual steps to know how to create container images easily.

RELEASE.md

@@ -9,10 +9,10 @@ The Kubespray Project is released on an as-needed basis. The process is as follo
 5. Create the release note with [Kubernetes Release Notes Generator](https://github.com/kubernetes/release/blob/master/cmd/release-notes/README.md). See the following `Release note creation` section for the details.
 6. An approver creates [new release in GitHub](https://github.com/kubernetes-sigs/kubespray/releases/new) using a version and tag name like `vX.Y.Z` and attaching the release notes
 7. An approver creates a release branch in the form `release-X.Y`
-8. The corresponding version of [quay.io/kubespray/kubespray:vX.Y.Z](https://quay.io/repository/kubespray/kubespray) and [quay.io/kubespray/vagrant:vX.Y.Z](https://quay.io/repository/kubespray/vagrant) docker images are built and tagged
+8. The corresponding version of [quay.io/kubespray/kubespray:vX.Y.Z](https://quay.io/repository/kubespray/kubespray) and [quay.io/kubespray/vagrant:vX.Y.Z](https://quay.io/repository/kubespray/vagrant) container images are built and tagged. See the following `Container image creation` section for the details.
 9. The `KUBESPRAY_VERSION` variable is updated in `.gitlab-ci.yml`
 10. The release issue is closed
-11. An announcement email is sent to `kubernetes-dev@googlegroups.com` with the subject `[ANNOUNCE] Kubespray $VERSION is released`
+11. An announcement email is sent to `dev@kubernetes.io` with the subject `[ANNOUNCE] Kubespray $VERSION is released`
 12. The topic of the #kubespray channel is updated with `vX.Y.Z is released! | ...`
 
 ## Major/minor releases and milestones
@@ -61,3 +61,23 @@ release-notes --start-sha <The start commit-id> --end-sha <The end commit-id> --
 
 If the release note file(/tmp/kubespray-release-note) contains "### Uncategorized" pull requests, those pull requests don't have a valid kind label(`kind/feature`, etc.).
 It is necessary to put a valid label on each pull request and run the above release-notes command again to get a better release note)
+
+## Container image creation
+
+The container image `quay.io/kubespray/kubespray:vX.Y.Z` can be created from Dockerfile of the kubespray root directory:
+
+```shell
+cd kubespray/
+nerdctl build -t quay.io/kubespray/kubespray:vX.Y.Z .
+nerdctl push quay.io/kubespray/kubespray:vX.Y.Z
+```
+
+The container image `quay.io/kubespray/vagrant:vX.Y.Z` can be created from build.sh of test-infra/vagrant-docker/:
+
+```shell
+cd kubespray/test-infra/vagrant-docker/
+./build vX.Y.Z
+```
+
+Please note that the above operation requires the permission to push container images into quay.io/kubespray/.
+If you don't have the permission, please ask it on the #kubespray-dev channel.