Reaper needs a static image filesystem to enable readOnlyRootFilesystem

[adejanovski]

When adding the following security context settings to Reaper, it fails to start complaining about access rights to /etc/cassandra-reaper:

          securityContext:
            capabilities:
              drop:
                - ALL
            runAsUser: 999
            runAsGroup: 999
            runAsNonRoot: true
            readOnlyRootFilesystem: true
            allowPrivilegeEscalation: false

We need to be able to run the Reaper pods with read only root FS.

Definition of Done

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1. Issues running the container with a read only root FS thelastpickle/cassandra-reaper#1506
   done +1
2. an emptyDir volume is mounted in both init/containers to host the config files in writeable form