fix docker extraction
fix docker extraction

[#117827631]

Signed-off-by: Yucheng Tu <84a3cda22a48c9e81f15e27384f15241ac826585@pivotal.io>

ggeorgiev/docker-image-resource,concourse/docker-image-resource,ggeorgiev/docker-image-resource,concourse/docker-image-resource,SillyMoo/docker-image-resource,barthy1/docker-image-resource,barthy1/docker-image-resource,SillyMoo/docker-image-resource