Allow docker image for cephfs provisioner to build the provisioner itself
Allow docker image for cephfs provisioner to build the provisioner itself

I've added a pre-step to build the cephfs-provisioner binary within docker, so this dockerfile could be used in e.g. quay automated builds.

Due to how vendoring is set up, the context of docker build has to be the root of the repository itself.

humblec/external-storage,kubernetes-incubator/external-storage,humblec/external-storage,kubernetes-incubator/external-storage,kubernetes-incubator/external-storage,humblec/external-storage