try platform hook

.platform/hooks/predeploy/00_build.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+
+echo installing yarn
+
+# install yarn
+sudo npm install yarn -g
+
+echo link
+
+# make sure it's in path
+sudo ln -s "$(npm bin --global)"/yarn /usr/bin/yarn
+
+echo try to build
+
+# build it
+yarn build