Fully specify bugfix version of Docker containers in Flink Playground docker-compose.yaml files

I recently noticed that we do not guarantee client-jobmanager compatibility among different bugfix versions of the same minor version (see https://github.com/ververica/sql-training/issues/8 for details).

In this case, a job submitted via a Flink 1.9.0 client could not be executed on a Flink 1.9.2 cluster.

For the playgrounds this can easily happen, because we build a custom Docker image (with a fixed bugfix version) and load the latest Flink images for the latest bugfix version of the same minor version.

We can avoid such problems by fixing the bugfix version of the Flink images that we run in the playground.