Ubuntu/Debian: Create a dataset for docker-ce

Beauregard Slim suggested this in #8301, saying:
    Apparently docker-ce automatically detects if /var/lib/docker is on
    ZFS and, without asking and with no changes to the default config,
    starts creating ZFS snapshots and clones of whatever dataset that
    directory is contained in (i.e. rpool/ROOT/ubuntu). I would
    recommend that the guide include creation of a docker data set
    exactly like it does for nfs

It is unclear to me if this behavior is specific to docker-ce or
applies to the standard docker packages shipped by the distro.

Debian-Stretch-Root-on-ZFS.md

@@ -130,6 +130,9 @@ Properties are inherited, if you want to create (for example) `rpool/var/lib` yo
     If this system will store local email in /var/mail:
     # zfs create                                            rpool/var/mail
 
+    If this system will use docker-ce (which creates snapshots by default):
+    # zfs create -o mountpoint=/var/lib/docker              rpool/var/docker
+
     If this system will use NFS (locking):
     # zfs create -o com.sun:auto-snapshot=false \
                  -o mountpoint=/var/lib/nfs                 rpool/var/nfs

Ubuntu-18.04-Root-on-ZFS.md

@@ -151,6 +151,9 @@ With ZFS, it is not normally necessary to use a mount command (either `mount` or
     If this system will store local email in /var/mail:
     # zfs create                                            rpool/var/mail
 
+    If this system will use docker-ce (which creates snapshots by default):
+    # zfs create -o mountpoint=/var/lib/docker              rpool/var/docker
+
     If this system will use NFS (locking):
     # zfs create -o com.sun:auto-snapshot=false \
                  -o mountpoint=/var/lib/nfs                 rpool/var/nfs