Make GCI_DOCKER_VERSION optional

fixes kubernetes#26813

This env var is only set in `config-test.sh` and trying to evaluate it with
`nounset` set will end up with an "unbound variable" error.

cluster/gce/gci/helper.sh

@@ -32,7 +32,7 @@ EOF
   fi
   if [[ ! -f "${KUBE_TEMP}/gci-docker-version.txt" ]]; then
     cat >"${KUBE_TEMP}/gci-docker-version.txt" << EOF
-${GCI_DOCKER_VERSION}
+${GCI_DOCKER_VERSION:-}
 EOF
   fi
 }