Note from the Docker guy: I look forward to reviewing this PR, I agree zfs support would be an awesome addition. However, note that this PR is not against the a

Note from the Docker guy: I look forward to reviewing this PR, I agree zfs support would be an awesome addition. However, note that this PR is not against the actual Docker repo but against a contributor's fork. I suppose it's because they don't yet feel it is ready to be contributed upstream. Except it's not marked as a fork in github so not sure if the usual PR system will work? In any case, we are happy to help if we can. Also note that we have merged experimental overlayfs support in the upcoming 1.4 release candidate.