SchemaRegistrySerializationSpec: flaky test

container startup:

```
[info] SchemaRegistrySerializationSpec:
[info] docs.scaladsl.SchemaRegistrySerializationSpec *** ABORTED *** (1 minute)
[info]   java.lang.RuntimeException: java.util.concurrent.ExecutionException: org.testcontainers.containers.ContainerLaunchException: Container startup failed for image confluentinc/cp-kafka:7.9.0
```