With docker-engine 1.10 tarring /var/lib/docker takes forever

[hannes-ucsc]

Pinning docker-engine to 1.9.1 prevents the problem.

There is a sparse file of 100G in /var/lib/docker. I think that's the cause. Even after 5min, the output .tar.gz is only 35M.

We should look into a ways to deal with archival of sparse files.