build.make: skip shellcheck if Docker is not available
build.make: skip shellcheck if Docker is not available

Not all environments have Docker. The simplifying assumption here is
that if the Docker command is available, it's also usable.

kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-lib-utils,kubernetes-csi/csi-driver-nfs,kubernetes-csi/external-attacher-csi,kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-lib-utils,kubernetes-csi/csi-lib-utils