fix: enable read-only root filesystem for trainer manager (#3119)

Signed-off-by: Sameer_yadav <159073326+Goku2099@users.noreply.github.com>

Author: Goku2099

manifests/base/manager/manager.yaml

@@ -24,6 +24,7 @@ spec:
         - name: manager
           image: ghcr.io/kubeflow/trainer/trainer-controller-manager
           securityContext:
+            readOnlyRootFilesystem: true
             allowPrivilegeEscalation: false
             runAsNonRoot: true
             capabilities: