Disable container privilege escalation.

Per Azure best practice, disable a container's ability to escalate its
privileges.

https://docs.microsoft.com/en-us/azure/aks/developer-best-practices-pod-security#secure-pod-access-to-resources

deploy/shared/migration.yaml

@@ -16,6 +16,8 @@ spec:
       containers:
       - name: migration
         image: $CONTAINER_IMAGE
+        securityContext:
+          allowPrivilegeEscalation: false
         command: [
           "/bin/sh", "-c"
         ]