Heroku release command bash wrapper (github#18743)

* Add tiny Bash wrapper to check for Node before attempting to execute Heroku release script

* Use the wrapper script for the Heroku release command

* Be explicit about exiting with Node's exit code

* If Node is missing, exit using that script's exit code

Author: JamesMGreene

Procfile

@@ -1,3 +1,3 @@
 web: NODE_ENV=production node server.js
 
-release: NODE_ENV=production node script/purge-redis-pages.js
+release: NODE_ENV=production script/release-heroku

script/release-heroku

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+
+# [start-readme]
+#
+# Light Bash wrapper for the Heroku release command, which sometimes fails
+# unexpectedly in staging environments when a Node installation is missing
+#
+# [end-readme]
+
+# Check for node but don't fail immediately if it's not present
+./script/check-for-node
+EXIT_STATUS=$?
+
+# If node is missing...
+if [[ "$EXIT_STATUS" -ne "0" ]]; then
+  # Fail hard if this is our Heroku production app or if Redis is configured
+  if [[ "$HEROKU_PRODUCTION_APP" == "true" || -n "$REDIS_URL" ]]; then
+    echo "Error: cannot execute the release script without Node.js, which is fatal."
+    echo "Exiting..."
+    exit $EXIT_STATUS
+  # Otherwise succeed with only a warning
+  else
+    echo "Warning: although Node.js is missing, it is non-critical."
+    echo "Exiting..."
+    exit 0
+  fi
+fi
+
+# Execute the release script and exit with its status
+node script/purge-redis-pages.js
+exit $?