docker: sync live ebuild with current upstream ebuild

These changes are not applicable to 1.3.3 so that ebuild is now a copy
instead of a symlink. This is in preparation for adding 1.4.0.
docker: add wrapper for detecting storage driver and bump to 1.4

The new dockerd wrapper script does its best to select between the btrfs
and overlay backends based on the filesystem mounted at /var/lib/docker.

The new 1.4 version will remain marked as ~amd64 for testing purposes
until we stabilize its dependencies, including Linux 3.18.x.
Merge pull request #1011 from marineam/docker

Add docker 1.4 and storage driver auto-detection for testing