Run more frequent docker prunes
Run more frequent docker prunes

Running the volume prune only once a day seemed to be insufficient as we
could still build up a lot of data. This also doubles the dangling prune
to every hour to be a bit more aggressive.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet