docker-engine: fix handling of vendor tree

docker-engine: fix handling of vendor tree

The latest docker-engine release changes the vendor tree slightly. The
Go tool will recognize a ./vendor directory and use it as a vendor tree
automatically, but only when run inside a valid GOPATH. This patch
adjusts how the GOPATH is built - now docker/docker is linked into a
blank tree, and the Go tool recognizes the ./vendor directory correctly.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>