Tighten docs publishing flow (#12572)

.travis/channel_restriction.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+#
+# Only proceed if we are on one of the channels passed in when calling this file
+#
+
+set -ex
+
+eval "$(ci/channel-info.sh)"
+
+for acceptable_channel in "$@"; do
+  if [[ "$CHANNEL" == "$acceptable_channel" ]]; then
+    exit 0
+  fi
+done
+
+echo "Not running from one of the following channels: $*"
+exit 1