build a rootfs for integration using docker

new docker image name is cloudfoundry/garden-ci; it has garden's test rootfs
baked in under /opt/warden/rootfs

Signed-off-by: Onsi Fakhouri <onsi@pivotallabs.com>
. /etc/seed if it exists on container creation

Signed-off-by: Onsi Fakhouri <onsi@pivotallabs.com>