hack/lib/util.sh: fix empty array expansion with bash 3.

hack/lib/util.sh

@@ -649,7 +649,9 @@ EOF
 # Determines if docker can be run, failures may simply require that the user be added to the docker group.
 function kube::util::ensure_docker_daemon_connectivity {
   IFS=" " read -ra DOCKER <<< "${DOCKER_OPTS}"
-  DOCKER=(docker "${DOCKER[@]}")
+  # Expand ${DOCKER[@]} only if it's not unset. This is to work around
+  # Bash 3 issue with unbound variable.
+  DOCKER=(docker ${DOCKER[@]:+"${DOCKER[@]}"})
   if ! "${DOCKER[@]}" info > /dev/null 2>&1 ; then
     cat <<'EOF' >&2
 Can't connect to 'docker' daemon.  please fix and retry.