Add env variable to Docker image for Memory Storage backend location

[emerkle826]

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Depends upon

• [x] #1487

Once https://github.com/thelastpickle/cassandra-reaper/issues/1487 is done, we need to add an env variable to the Docker image to set the location for the Memory Storage backend persistence location.

Definition of Done

• [ ] The Dockerfile and config file are wired up to configure the persistent storage path for the Memory store