Fix the linux image post_push script
Fix the linux image post_push script

Previously the script failed to compute the tree SHA because the
`git ls-tree` in the Docker Cloud environment didn't like the
path `../go`. It works fine as `go` if we `cd ..` first.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit