Comment 0 for bug 1879473

[Rationale]
ZFS module of docker creates many snapshots under /var/lib/<ID>. These snapshots are processed by zsys and lead to timeout.

This patch migrates the snapshots created by docker under the persistent dataset /var/lib/docker/ so they are scanned and then ignored by zsys.

[Test Case]
1. On a machine with docker installed, create a docker container:
$ docker run ubuntu
2. Verify that docker created snapshots under /var/lib/<ID> with zfs list
3. Install the new version
4. Verify that the snapshots have been migrated to /var/lib/docker/
5. Verify that docker is still working:
$ docker run ubuntu

[Regression Potential]
The change only affects the postinst.
The risk is that dataset are not migrated or partially migrated. Then new snapshots are created on a different location and containers created or run from an existing image won't start.