Keep it robust against restarting

[gordonwatts]

The following things would need to be cached somewhere they don't disappear between invocations:

• The mongo db
• The results directory attached to xrootd.

As a bonus:

• The cpp cache directory

Perhaps we can just copy the docker config to make sure this works correctly.

[gordonwatts]

The results directory is now done. As is the cache directory, btw (though currently, that isn't a very big deal).