Add snapshot init containers

[ferranbt]

Right now we have the concept of artifact containers, they download the artifact if not found locally or checsum does not match. We have to extend this to another kind of init containers, the snapshot containers. These containers mount the data volumes and run some action and are only executed if the data volume is empty.

Since these containers can be arbitrary (i.e. imagine the Fetch utility or the Tempo snapshot command), we need a way to wrap the logic of these containers around some marker files in the docker volume to signal whether the action gets executed or not.