chore(images): pin service image updates

Replace floating service images with reviewable tags or digests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

edu_master/k8s/session-keeper.yaml

@@ -17,7 +17,7 @@ spec:
     spec:
       initContainers:
         - name: wait-redis
-          image: redis:alpine
+          image: redis:8.0.3-alpine
           command:
             - /bin/sh
             - -ec