Store hashes of docker images used in experiment

[denkv]

Sometimes we need to check exactly which images were used in a particular experiment.

We can store image names and hashes for every container created during an experiment in that experiment's result model. They can be also easily displayed in experiment result pages.

[MichaelRoeder]

The information can be collected and added to the result model easily by the platform controller. However, we need to define the properties that we would like to use for that. In this context, we may want to answer the following questions:

• Should we distinguish between containers that were part of the benchmark and containers that were part of the system?
• Should we add a count for the number of containers that have been created?
• Should we introduce a new data type to express that the literals are Docker image hashes and not simply Strings?