Documentation: Add note about pinning the container after release

Add note that one the patches after branching should
pin the container to the current LTS version so the
CI on that branch remains stable as long as possible.

Signed-off-by: Ales Musil <amusil@redhat.com>
Signed-off-by: Dumitru Ceara <dceara@redhat.com>

Documentation/internals/release-process.rst

@@ -64,6 +64,10 @@ Scheduling`_ for the timing of each stage:
    branch.  Features to be added to release branches should be limited in scope
    and risk and discussed on ovs-dev before creating the branch.
 
+   In order to keep the CI stable on the new release branch, the Ubuntu
+   container should be pinned to the current LTS version in the Dockerfile
+   e.g. registry.hub.docker.com/library/ubuntu:22.04.
+
 3. When committers come to rough consensus that the release is ready, they
    release the .0 release on its branch, e.g. 25.09.0 for branch-25.09.  To
    make the actual release, a committer pushes a signed tag named, e.g.