Revisit backup

[davidsowerby]

#38 provided a backup, but it is clumsy, and requires a lot of CLI input for such a simple operation.

Maybe would have been better just mounting a host volume on to Jenkins.
Initial attempt was to have the backup process/service running permanently, as a linked container, but could not get that to work (it would work only on the first pass - it probably needed setting up properly as a Service, and have some way of keeping the container 'live')